Tabular silver halide emulsions with ledges

A photographic emulsion is disclosed containing tabular silver halide grains having opposed major faces and ledges of relatively reduced thickness extending laterally beyond at least one of said major faces.

FIELD OF THE INVENTION 
This invention relates to photographic emulsions. More specifically, the 
invention relates to tabular grain silver halide emulsions. 
BACKGROUND OF THE INVENTION 
Photographic silver halide emulsions are dispersions of radiation sensitive 
silver halide microcrystals, referred to as grains, capable of forming a 
latent image. Photographic silver halides exclude silver fluoride, which 
is water soluble, and silver iodide, which, though highly useful in minor 
proportions, as a major grain component does not efficiently form 
developable latent images. Although photographic silver halide emulsions 
prepared by single jet precipitation techniques have been long known to 
contain some tabular grains, the photographic advantages offered by the 
presence of tabular grains in silver halide emulsions was not appreciated 
until relatively recently. 
Depending upon the intended photographic application and the halide content 
of the tabular grains, tabular grain emulsions have been recently 
disclosed in which tabular grains of (i) 0.5 micrometer (hereinafter 
designated .mu.m) or less in thickness, more typically 0.3 .mu.m or less 
in thickness, and optimally less than 0.2 .mu.m in thickness (ii) having 
an average aspect ratio of at least 5:1, more typically greater than 8:1, 
and (iii) accounting for greater than 35 percent, more typically greater 
than 50 percent, of the total grain projected area of the emulsion have 
been disclosed. Disclosed advantages have included increased speed, 
improved developability, improved speed-granularity relationships, 
increased sharpness, increased blue and minus blue speed separations, 
higher developed silver covering power of fully forehardened emulsions, 
reduced crossover in dual coated radiographic elements, higher transferred 
image densities at reduced silver coverages in image transfer photography, 
and reduced thermal variance and rereversal in direct reversal 
applications. Illustrative of high and intermediate aspect ratio tabular 
grain emulsions, their methods of preparation, and their photographic 
advantages are the following: 
(T-1) Wilgus et al U.S. Pat. No. 4,434,226, 
(T-2) Kofron et al U.S. Pat. No. 4,439,520, 
(T-3) Daubendiek et al U.S. Pat. No. 4,414,310, 
(T-4) Abbott et al U.S. Pat. No. 4,425,425, 
(T-5) Wey U.S. Pat. No. 4,399,215, 
(T-6) Solberg et al U.S. Pat. No. 4,433,048, 
(T-7) Dickerson U.S. Pat. No. 4,414,304, 
(T-8) Mignot U.S. Pat. No. 4,386,156, 
(T-9) Jones et al U.S. Pat. No. 4,478,929, 
(T-10) Evans et al U.S. Pat. No. 4,504,570, 
(T-11) Maskasky U.S. Pat. No. 4,400,463, 
(T-12) Wey et al U.S. Pat. No. 4,414,306, 
(T-13) Maskasky U.S. Pat. No. 4,435,501, 
(T-14) Abbott et al U.S. Pat. No. 4,425,426, 
(T-15) Research Disclosure, Vol. 232, Aug. 1983, Item 23212, and 
(T-16) Research Disclosure, Vol. 225, Jan. 1983, Item 22534. 
Research Disclosure is published by Kenneth Mason Publications, Ltd., 
Emsworth, Hampshire P010 7DD, England. 
While initial investigations of tabular grain emulsions focused on serving 
predominantly higher speed photographic applications, more recently 
attention has been focused on relatively slower speed emulsions. 
Daubendiek et al U.S. Ser. Nos. 790,692 and 790,693, both filed Oct. 23, 
1985, refiled Aug. 1, 1986, as U.S. Ser. Nos. 891,803 and 891,804, 
respectively, all commonly assigned, disclose the utility of small, thin 
tabular grain emulsions in color photograpay. Specifically, the utility is 
disclosed in blue and minus blue recording layers of color photographic 
elements of emulsions having tabular grain mean diameters in the range of 
from 0.2 to 0.55 .mu.m, wherein the grains have average aspect ratios 
greater than 8:1 and account for greater than 50 percent of the total 
grain projected areas. 
A unifying theme running through these various tabular grain emulsion 
disclosures is the importance of having the tabular grains account for a 
high proportion of the total grain projected area, where the term 
"projected area" is used in the same sense as the terms "projection area" 
and "projective area" commonly employed in the art; see, for example, 
James and Higgins, Fundamentals of Photographic Theory, Morgan and Morgan, 
New York, p. 15. These disclosures also emphasize the importance of 
increasing average aspect ratios, where aspect ratio is defined as the 
ratio of the diameter of a tabular grain to its thickness. The diameter of 
a tabular grain is the diameter of a circle whose area is equal to the 
projected area of the tabular grain. It is generally recognized and 
accepted that to the extent (i) the average aspect ratio of a tabular 
grains and (ii) the percentage of the total grain projected area accounted 
for by tabular grains, can be increased, the photographic properties of 
the tabular grain emulsions can be improved. 
All photographically useful silver halides form grains--i.e., 
microcrystals--of a cubic crystal lattice structure. The silver halide 
grains are bounded by cubic or {100} crystallographic planes, octahedral 
or {111} crystallographic planes, and/or rhombic dodecahedral or {110} 
crystallographic planes, the latter occurring only rarely. {100} 
(occasionally also referred to as {200}), {111}, and {110} are Miller 
index assignments of the grain crystal faces. Regular grains bounded 
entirely by {100} crystal faces form regular cubes, regular grains bounded 
by {111} crystal faces form regular octahedra, and regular grains bounded 
by {110} crystal faces form regular rhombododecahedra. 
It has been recently observed that there are four additional families of 
crystallographic planes that can bound cubic crystal lattice silver halide 
grains: 
(1) Maskasky U.S. Ser. No. 771,861, titled SILVER HALIDE PHOTOGRAPHIC 
EMULSIONS WITH NOVEL GRAIN FACES (1), discloses emulsions containing 
silver halide grains bounded by hexoctahedral crystallographic planes. 
Hexoctahedral crystallographic planes satisfy the Miller index assignment 
{hkl}, wherein h, k, and l are integers greater than zero, h is greater 
than k, and k is greater than l. Most commonly h is 5 or less. 
(2) Maskasky U.S. Ser. No. 772,228, titled SILVER HALIDE PHOTOGRAPHIC 
EMULSIONS WITH NOVEL GRAIN FACES (2), discloses emulsions containing 
silver halide grains bounded by tetrahexahedral crystallographic planes. 
Tetrahexahedral crystallographic planes satisfy the Miller index 
assignment {hh0}, wherein 0 is zero, h and k are integers greater than 0 
and different from each other. Most commonly h and k are no greater than 
5. 
(3) Maskasky U.S. Ser. No. 772,229, titled SILVER HALIDE PHOTOGRAPHIC 
EMULSIONS WITH NOVEL GRAIN FACES (3), discloses emulsions containing 
silver halide grains bounded by trisoctahedral crystallographic planes. 
Trisoctahedral crystallographic planes satisfy the Miller index assignment 
{hhl}, wherein h and l are integers greater than zero and h is greater 
than l. Most commonly h is no greater than 5. 
(4) Maskasky U.S. Ser. No. 772,230, titled SILVER HALIDE PHOTOGRAPHIC 
EMULSIONS WITH NOVEL GRAIN FACES (4), discloses emulsions containing 
silver halide grains bounded by icositetrahedral crystallographic planes. 
Icositetrahedral crystallograpaic planes satisfy the Miller index 
assignment {hll}, wherein h and l are integers greater than zero and h is 
greater than l. Most commonly h is no greater than 5. 
These patent applications were all filed Sept. 3, 1985, and refiled as U.S. 
Ser. Nos. 881,768, 881,769, 882,112, and 882,113, on July 3, 1986, and are 
all commonly assigned. The novel crystallographic faces were made possible 
by finding grain growth modifiers capable of reducing the rate of growth 
of the crystal face desired, since it is the slowest growing crystal faces 
that bound the grains and give them their surfaces. 
(5) Maskasky U.S. Ser. No. 772,271, filed Sept. 3, 1985, commonly assigned, 
titled SILVER HALIDE PHOTOGRAPHIC EMULSIONS WITH NOVEL GRAIN FACES (5) 
discloses tabular grain emulsions having opposed major octahedral or 55 
111} faces which are ruffled by the deposition of silver halide thereon. 
By the use of grain growth modifiers ruffling deposits capable of forming 
any of the remaining six families of crystallographic planes possible with 
cubic crystal lattice silver halide grains can be formed. 
SUMMARY OF THE INVENTION 
In one aspect this invention is directed to a photographic emulsion 
comprised of tabular silver halide grains having opposed major faces. The 
emulsions are characterized in that tabular grains are present having 
ledges of relatively reduced thickness extending laterally beyond at least 
one of said major faces. 
The advantages of the present invention are that the known desirable 
properties of tabular grain emulsions for photographic applications can be 
further enhanced. The ledge extensions of the tabular grains increase the 
projected area of the grains. In addition, since the thickness of the 
ledges is less than that of the tabular grains measured between the 
opposed major faces, it is apparent that the effective aspect ratio of the 
tabular grains is increased. Stated more succinctly, the present invention 
can be employed to enhance the tabularity of photographic silver halide 
emulsions.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In conventional photographic tabular grain silver halide emulsions the 
majority of tabular grains present appear in plan view to have opposed 
major faces which correspond in shape to a hexagon or an equilateral 
triangle. While the grains have opposed parallel major crystal faces, the 
faces are superimposed so that only one major face is visible. 
FIG. 1 shows a conventional tabular grain 100 presenting a major face 101 
of a hexagonal shape. FIG. 2 shows a conventional tabular grain 200 
presenting a major face 201 of a triangular shape. FIGS. 3 and 4 
illustrate tabular grains from emulsions of this invention, which are 
formed from the conventional tabular grains 100 and 200, respectively. 
It is readily apparent that the tabular grain 300 in FIG. 3 differs from 
the grain 100 of FIG. 1 in that it presents a larger projected area and 
exhibits a distinctive shape. The grain 300 is bounded by twelve edges 
301a, 301b, 301c, 301d, 301e, 301f, 301g, 301h, 301i, 301j, 301k, and 
301l, which appear distinctly linear. Completing the periphery of the 
grain as viewed in plan are six edges 307a, 307b, 307c, 307d, 307e, and 
307f, which sometimes appear linear, but frequently appear uneven, as 
shown. In some hexagonal tabular grains according to this invention the 
307 series edges are not present. Instead of having a 307 series edge 
separating two 301 series edges the 301 series edges intersect forming a 
coign at their intersection. 
There is also a difference when viewed under a reflected light microscope 
that FIGS. 1 and 3 do not capture, since they do not show the hue of the 
grains. It is known that conventional tabular grains by reason for the 
fractional .mu.m spacings between their major faces as well as the 
parallel relationship of the major faces exhibit brilliant colors of 
uniform hue. The tabular grain 100 can be of any visible hue, depending 
upon its exact thickness. The relationship between tabular grain thickness 
and the wavelength of reflected light is discussed in Research Disclosure, 
Vol. 253May 1985, Item 25330. When the tabular grain 100 is of uniform 
composition throughout, as is usually the case, it exhibits one visible 
hue. The hue is often a highly saturated primary color. 
Viewed under a microscope the grain 300 similarly exhibits a single hue 
within the hexagonal area bounded by edges 303a, 303b, and 303c and 
alternating edges 305a, 305b, and 305c. However, in the areas lying 
laterally beyond the hexagonal area, hereinafter referred to as shelves or 
ledges, a distinctly different hue is observed. In some instances the 
triad of ledges 309a, 309b, and 309c, lying adjacent the hexagonal area 
edges 303a, 303b, and 303c, respectively, are of a different hue than the 
triad of ledges 311a, 311b, and 311c lying adjacent the hexagonal area 
edges 305a, 305b, and 305c, respectively. However, the ledges within each 
triad are of identical hue. This indicates that the ledges within each 
triad are all of the same uniform thickness and that this thickness is 
different from the thickness of the hexagonal area of the grain. 
Upon direct viewing or in color photomicrographs both triads of ledges are 
visible because of the hue differentiation of the hexagonal area of the 
tabular grain. In electron photomicrographs, the hexagonal area edges 
303a, 303b, and 303c are clearly visible, indicating that these edges on 
the viewed side of the tabular grain. On the other hand, the hexagonal 
area edges 305a, 305b, and 305c are not visible, indicating that they are 
edges on the remote side of the tabular grain. 
From these observations it is apparent that edges 303a, 301c, 307b, 301d, 
303b, 301g, 307d, 301h, 303c, 301k, 307f, and 301l are the boundaries of 
the upper major face of the tabular grain 300 while the edges 301a, 307a, 
301b, 305a, 301e, 307c, 301f, 305b, 301i, 307e, 301j, and 305c define the 
boundaries of the lower major face of the tabular grain 300. The two major 
faces are identical, but differ by an angle of 60.degree. in their edge 
orientations. Each major face is laterally extended by one triad of 
ledges. Electron microscopic examination of grains tipped sufficiently to 
permit edge viewing confirm the presence of ledges of relatively uniform 
thickness and of less thickness than the spacing between the grain major 
faces. 
It is similarly apparent that the tabular grain 400 in FIG. 4 differs from 
the grain 200 of FIG. 2 in that it presents a larger projected area and 
exhibits a distinctive shape. The grain exhibits edges 401a, 401b, and 
401c that define a triangular area 403 corresponding to the major face 
201. This area is of one uniform hue, indicating that it is of uniform 
thickness. Lying along each of the triangle defining edges are ledges 
405a, 405b, and 405c. These ledges are all of the same hue, which differs 
from that of the triangular area, indicating that the ledges are of 
uniform thickness and of a thickness different from that of the triangular 
area. Since the edges 401a, 401b, and 401c are all visible and since no 
grains of this shape have been observed in which these edges are not 
visible, it is apparent that the ledges do not form extensions of either 
of the two triangular major faces of these grains. 
ln viewing tabular grains with triangular major faces and ledges in 
emulsions according to this invention, it is noted that at an early stage 
of formation the ledges can appear as discontinuous protrusions along the 
equilateral triangle edges. with further growth the ledges become 
continuous along an edge. Like the linear 301 series edges of the grain 
300 linear edges 409a, 409b, 409c, 409d, 409e and 409f are noted to 
diverge from the coigns 407a, 407b and 407c of the triangular area 403. 
The edges 411a, 411b, and 411c initially appear uneven, but with continued 
growth often appear linear and parallel to the triangle edges 401a, 401b, 
and 401c, respectively. It is possible to grow the 411 series edges out of 
existence. In other words the two 409 series edges forming a ledge can 
intersect forming a coign at their intersection. This has been observed 
for relatively smaller projected area grains, but should be possible with 
continued ledge growth for larger projected area grains as well. 
The ledges of the tabular grain emulsions of this invention preferably 
account for at least 5 percent of the total projected area of the tabular 
grains having ledges. While it is believed that ledge projected areas can 
account for 50 percent of the total projected area of a tabular grain 
having ledges, tabular grains having ledge projected areas in the range of 
from about 5 to 20 percent based on the total projected area of tabular 
grains having ledges are most conveniently prepared. 
Emulsions satisfying the requirements of this invention can be prepared by 
growing ledges on the tabular grains of any conventional photographic 
silver halide emulsion containing hexagonal or triangular projected area 
tabular grains. For example, emulsions according to this invention can be 
prepared by growing shelves or ledges on any of the intermediate and high 
aspect ratio tabular grain emulsions disclosed in references T-1 through 
T-17, cited above, except T-8, which discloses only square and rectangular 
projected area tabular grains. 
At least 35 percent of the total grain projected area of emulsions 
according to the invention are accounted for by tabular grains having 
ledges. Usually, instead of 35 percent, tabular grains having ledges 
account for at least 50 percent and preferably at least 70 percent of the 
total grain projected area. 
In general the tabular grain emulsions of this invention satisfying the 
projected area requirements indicated above are those in which the tabular 
grains having ledges counted in satisfying the projected area percentages 
have a thickness between their major faces of 0.5 .mu.m or less, 
preferably 0.3 .mu.m or less, and optimally 0.2 .mu.m or less. Tabular 
grains of such thickness typically have an average aspect ratio of greater 
than 5:1, preferably greater than 8:1, and optimally at least 12:1. 
Conventional tabular grain emulsions are known to have aspect ratios 
ranging up to 100:1 and, in some instances, up to 200:1. Optimum average 
aspect ratios are typically in the range of from 12:1 to about 75:1 for 
silver bromide and bromoiodide emulsions. The addition of ledges should 
permit these average aspect ratios to be more readily satisfied or even 
increased. 
In determining the aspect ratio of tabular grains having ledges the 
projected area contributed by the ledges is included in calculating the 
grain diameter, but the tabular grain thickness remains the distance 
between the major faces of the grain and does not take into account the 
thinning of the tabular grains attributable to the presence of the ledges. 
The reason for this basis of definition is that grain thickness is most 
readily determined by grain shadow lengths, which do not lend themselves 
to ledge thickness determinations. It therefore must be kept in mind that 
a tabular grain having ledges according to this invention having a 
calculated aspect ratio of 12:1, for example, actually has a somewhat 
higher aspect ratio than a conventional tabular grain lacking ledge 
extensions and also having a calculated aspect ratio of 12:1. 
The preferred photographic emulsions according to this invention are those 
in which tabular silver bromide or silver bromoiodide grains with ledges 
and having a thickness of 0.3 .mu.m or less (optimally 0.2 .mu.m or less) 
have an average aspect ratio of greater than 8:1 (optimally at least 12:1) 
and account for greater than 50 percent (optimally greater than 70 
percent) of the total grain projected area. In these emulsions the ledges 
account for at least 5 percent (optimally 5 to 20 percent) of the 
projected area of the tabular grains having ledges. 
The composition of the tabular grains having ledges can correspond to that 
of the tabular grains of known photographic silver halide emulsions. 
Tabular grains having ledges consisting essentially of silver bromide are 
readily formed. Silver bromoiodide tabular grain emulsions according to 
this invention can be formed readily also, particularly where the iodide 
concentration is maintained at about 6 mole percent or less, based on 
silver. 
The ledges of the tabular grains are grown onto host tabular grains. The 
ledges can be of the same composition as the host tabular grains. The host 
tabular grains as well as the ledges grown on them can be of either 
uniform composition or nonuniform composition. For example, (T-6) Solberg 
et al, cited above, discloses higher iodide peripherally than in a central 
grain region while (T-12) Wey et al, cited above, discloses silver 
chlorobromide in an annular tabular grain region. Where the host tabular 
grains are themselves of nonuniform composition, it is generally most 
convenient to deposit ledges at least initially of a composition similar 
to that of the peripheral edges initially presented by the host tabular 
grains. It is specifically contemplated to vary the composition of the 
ledges as they are being formed. For example, although the techniques 
disclosed by (T-13) Maskasky, cited above, have not been observed to 
create ledges, these techniques can be used to extend or decorate 
epitaxially the ledges following initial formation by the techniques of 
this invention. The teachings of (T-13) Maskasky for controlled site 
epitaxial depositions are entirely compatible with tabular grains having 
ledges according to this invention. 
Processes by which ledges can be grown on host tabular grains are 
illustrated in the examples below. In general ledge growth can be 
undertaken under conventional silver halide precipitation conditions, 
including grain ripening conditions, in the presence of a suitable growth 
modifier. Azaindene, particularly tetraazaindene grain growth modifiers, 
such as 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindenes, have been found to be 
effective. Fortunately, these azaindenes are known to be useful 
photographic antifoggants and stabilizers and, in certain instances, 
sensitizers. Therefore, the azaindene grain growth modifiers can, if 
desired, be left in the emulsions after ledge formation and serve further 
useful purposes in subsequent photographic uses of the emulsions. 
The features of the emulsions so far discussed can be readily verified by 
observation and in no way depend upon any particular theoretical 
explanation. It is therefore neither intended nor necessary to depend on 
any particular theory to account for or describe the emulsions of this 
invention. Nevertheless, the observations of this invention are compatible 
with accepted theories as to the structure of photographically useful 
tabular silver halide grains and suggest refinements and extensions of 
these theories, which have been at least partially corroborated by further 
original investigations. Therefore, the following explanation is offered 
to provide not only a better insight into the probable structure of the 
tabular grains, but also a better insight into why and how they are 
formed. These insights should be useful to those skilled in the art in 
later investigations of these and derivative tabular grain emulsions. 
FIG. 5 presents an isometric view of the tabular grain 100 shown in FIG. 1, 
but with the thickness of the grain exaggerated for ease of illustration. 
Prior to this invention tabular silver bromide grains have been grown to 
sizes larger than those useful in photography and reported to have the 
appearance shown in FIG. 5. The grain 100 as shown consists of three 
superimposed strata 103, 105, and 107. The stratum 107 lies adjacent the 
upper major face 101 while the lower stratum 103 lies adjacent the 
parallel, opposed major face, not visible. A crystallographic twin plane 
109 separates the strata 103 and 105 while a second crystallographic twin 
plane 111 separates the strata 105 and 107. Three edges of the strata 103 
and 105 each form a reentrant angle of intersection of 141.degree. while 
three alternate edges of these strata each form a nonreentrant angle of 
intersection of 219.degree.. The strata 105 and 107 form similar angles of 
intersection, but oriented so that each reentrant angle of intersection of 
strata 105 and 107 lies above a nonreentrant angle of intersection of the 
strata 103 and 105 and vice versa. Thus, joining corresponding hexagonal 
major face edges there are strata edges forming one reentrant angle of 
intersection and one nonreentrant angle of intersection. It is generally 
accepted that the high aspect ratios of tabular grains is accounted for by 
the silver halide edge deposition preference created by the reentrant 
angles of intersection as compared to deposition on the major faces of the 
grains. 
In original observations of conventional silver bromide tabular grain 
emulsions it has been confirmed that most tabular grains present hexagonal 
projected areas and that most of these grains contain two twin planes. As 
is well recognized in the art a significant proportion of tabular grains 
present equilaterally triangular projected areas. On closer inspection 
many of the triangular projected areas are in fact hexagonal, but with 
three of the alternate edges of the hexagon being relatively restricted. 
For purpose of this discussion a tabular grain having a triangular 
projected area is defined as any grain having three major face edges more 
than an order of magnitude (10.times.) longer than any other edge of the 
major face. Using this definition it was noted that the common tabular 
grains encountered in sample conventional tabular grain silver bromide 
emulsions were as follows: 
Grain Category I--Hexagonal projected area tabular grains containing an 
even number of twin planes (typically &gt;80 percent of the grains); 
Grain Category II--Triangular projected area tabular grains containing an 
odd number of twin planes (typically in the order of about 10 percent of 
the grains); 
Grain Category III--Triangular projected area tabular grains containing an 
even number of twin planes (typically in the order of about 1 to 2 percent 
of the grains); and 
Grain Category IV--Hexagonal projected area tabular grains containing an 
odd number of twin planes (typically in the order of about 1 percent of 
the grains). 
Miscellaneous--A variety of grain shapes, including most notably tabular 
grains of trapezoidal and double trapezoidal projected areas. (For a 
discussion of trapezoidal projected area tabular grains, attention is 
directed to Maskasky U.S. Ser. No. 811,132, filed Dec. 19, 1985, titled A 
PROCESS FOR PRECIPITATING A TABULAR GRAIN EMULSION IN THE PRESENCE OF A 
GELATINO--PEPTIZER AND AN EMULSION PRODUCED THEREBY, commonly assigned.) 
While the proportions of the various grains can vary appreciably from one 
emulsion to the next, the relative order of occurrence is considered less 
likely to vary. 
When a tabular grain is being grown having two parallel twin planes, which 
is believed to be the minimum number of twin planes necessary in most 
instances to achieve high aspect ratios (greater than 8:1), an additional 
twin plane sometimes forms. The third twin plane predisposes the tabular 
grain to form a triangular rather than a hexagonal projected area. This 
can be appreciated by reference to FIG. 6, wherein a tabular grain 500 is 
shown having a hexagonal major face 501 and an opposed parallel hexagonal 
major face, which is not visible. The tabular grain consists of four 
superimposed strata 503, 505, 507, and 509. Separating adjacent strata are 
twin planes 511, 513, and 517. The edges of the strata form reentrant and 
nonreentrant angles of intersection similarly as the tabular grain 100, 
but with an important difference. It is to be noted that as shown the 
strata edges joining the shorter hexagonal major face edges form two 
reentrant angles of intersection, whereas the strata edges joining the 
longer hexagonal major face edges form only one reentrant angle of 
intersection. Based on previously accepted theories of tabular grain 
growth, the two to one ratio of reentrant angles of intersection should 
cause the strata edges joining the shorter major face edges to grow much 
more rapidly than the strata edges joining the longer major face edges. 
The result is that the shorter major face edges become progressively 
shorter as grain growth continues, and the hexagonal projected area of the 
tabular grain becomes a triangular projected area in accordance the 
definition provided above. 
The foregoing mechanism of triangular projected area tabular grain 
formation is supported by the relative frequencies of the various grain 
categories listed above. Specifically, it is believed that a few of the 
grains in Grain Category I experience an additional twinning event that 
moves them immediately into Grain Category IV. There are few grains in 
Grain Category IV, since these grains are in rapid growth transition to 
Grain Category II. Grain Category III may result from the strata forming 
the major faces exhibiting pronounced differences in their thicknesses, 
resulting in an asymmetry in the reentrant angles of intersection of 
alternate edges. 
The observation and categorization of tabular grains according to even or 
odd numbers of twin planes is an original observation, whereas the 
attribution of rapid edge growth in tabular grains to reentrant angles of 
strata edge intersections is in accordance with accepted theories. 
However, from further observations, discussed below, it is now believed 
that a more important determinant to rapid edge growth of tabular silver 
halide grains than the reentrant angle of interaction of strata edges is 
the angle which a stratum edge makes with the major face of the tabular 
grain. A stratum edge can by intersecting a major face at an angle of 
70.5.degree. form an acute lip or by intersecting a major face at an angle 
of 109.5.degree. form an obtuse lip. 
It is believed that it is the difference in surface crystallographic planes 
present at the apex of acute lips and obtuse lips that make ledge growth 
on tabular grains according to this invention possible. This can best be 
appreciated by reference to FIGS. 7A and 7B, which are enlarged sections 
of the tabular grain 300 in FIG. 3. As shown in these figures the tabular 
grain 300 has a first major face 701 and a second major face 703. The 
major faces, like those of most conventional tabular grains, lie in 
parallel octahedral (i.e., {111}) crystallographic planes. The tabular 
grain consists of strata 705, 707, and 709 lying between the major faces. 
Strata 705 and 707 are separated by a twin plane 711 while strata 707 and 
709 are separated by a twin plane 713. 
It is generally believed that all of the strata edge surfaces in 
conventional tabular grains as well as the major faces lie in {111} 
crystallographic planes. The strata edges of the host tabular grain onto 
which the ledges are grown are indicated by dashed lines 715 in FIGS. 7A 
and 7B. Extending laterally beyond the host tabular grain edge 715 in FIG. 
7A is an upper ledge 717 formed by strata 707 and 709. The upper surface 
of the upper ledge forms an extension of the upper major face 701; 
however, the lower surface of the upper ledge does not extend below the 
twin plan 711. The lower ledge 719 in FIG. 7B is of similar structure, its 
lower surface forming an extension of the major face 703. The lower ledge 
does not extend above the twin plane 713. 
It is believed that ledge growth in the form shown in FIGS. 7A and 7B is 
made possible by the host tabular grain edge 715 forming in FIG. 7A an 
obtuse lip 721 with the major face 703 and an acute lip 723 with the major 
face 701 and in FIG. 7B an obtuse lip 725 with the major face 701 and an 
acute lip with the major face 727. If host tabular grain {111} strata 
edges represented by 715 intersected the {111} major faces of the host 
tabular grains without any other crystal face being present at the grain 
surface, then it would be immaterial whether obtuse or acute lips were 
formed. However, it is well known that silver halide at the corners of 
grains is more readily solubilized than silver halide on flat grain faces, 
and it is further a common observation that silver halide grains exhibit 
rounding at the grain corners. It is believed that apices of the acute 
lips are rounded to reveal cubic or {100} crystal faces as well as 
icositetra-hedral or {hll} crystal faces. At the same time the apices of 
the obtuse lips are rounded to reveal rhombic dodecahedral or {110} 
crystal faces as well as trisoctahedral or {hhl} crystal faces. In the 
foregoing Miller index assignments h and l are both integers greater than 
zero and h is greater than l. Although h is not theoretically limited, it 
is typically 5 or less. 
It has been discovered that by employing a growth modifier capable of 
slowing the rate of silver halide deposition on trisoctahedral or {hhl} 
crystal faces it is possible to arrest the lateral growth of the tabular 
grain strata at their obtuse lips. It is believed that the obtuse lips 
grow only slightly to form trisoctahedral or {hhl} crystal faces, shown as 
faces 727 and 729 in FIGS. 7A and 7B, respectively. For example, the angle 
which the host tabular grain initially forms at its obtuse lips is 
109.5.degree.. When that angle is increased slightly to 136.7.degree.a 
{551} trisoctahedral crystal face is presented. By employing a grain 
growth modifier that adsorbs selectively to a {551} crystal face, the 
further deposition of silver halide on this crystal face, once formed, is 
arrested, and the {551} crystal face remains as a part of the final grain 
topography. Note that it is important that a growth modifier be employed 
which adsorbs selectively to trisoctahedral crystal faces as opposed to 
icositetrahedral or cubic crystal faces. 
Turning to FIG. 8, the sectional detail shown reveals ledge 405a to extend 
laterally beyond the major face 403 of the grain. The boundary of the host 
grain onto which the ledges were grown is shown by dashed line 801. The 
important difference between the hexagonal projected area tabular grains 
of FIGS. 3, 5, 7A, and 7B on the one hand and the tabular grains of FIGS. 
4 and 8 on the other hand, is that the latter grains contain three twin 
planes 803, 805, and 807 separating four strata 809, 811, 813, and 815 
rather than two parallel twin planes. This results in the triangular 
projected area tabular grains presenting obtuse lips at each of the edges 
of strata adjacent their major faces. Thia allows an adsorbed growth 
modifier to arrest the lateral growth of strata 809 and 815 adjacent the 
major faces. These two strata grow laterally only a negligible extent 
before forming trioctahedral crystal faces, indicated at 817 and 819. The 
interior strata 811 and 813 remain free to grow laterally and do so to 
form the ledge 405a. 
In the illustrative grains shown the strata forming the grains are all of 
uniform thickness. In this circumstance the ledges formed by the hexagonal 
projected area grains are two thirds the thickness of the host tabular 
grain while the ledges formed by the triangular projected area grains are 
only one half the thickness of the host tabular grain. In actuality the 
intervals between twinning events can vary so that strata of differing 
thicknesses can be formed within a single grain. It is believed, but not 
proven, that tabular grains having regular hexagon projected areas have at 
least symmetrical, if not identical strata thicknesses, while hexagonal 
projected area tabular grains with alternate triads of longer and shorter 
edges may exhibit dissimilar strata thicknesses. 
Apart from the features described above, the tabular grain emulsions of 
this invention include features corresponding to those known in 
conventional tabular grain emulsions. The teachings of references T-1 
through T-7 and T-9 through T-17 are here incorporated by reference to 
show conventional features, such as dispersing media (including peptizers 
and binders), vehicle hardening, chemical sensitization, spectral 
sensitization, emulsion blending, and varied addenda, such as antifoggants 
and stabilizers, and coating aids. Research Disclosure, Vol. 176, Dec. 
1978, Item 17643, is also incorporated by reference to show conventional 
emulsion features. The emulsions can be employed in photographic elements, 
exposed, and processed in any conventional matter, also illustrated by 
these references. 
In addition to conventional dispersing media it is contemplated to employ 
gelatino-peptizers containing less than 30 micromoles of methionine per 
gram. Such gelatino-peptizers can be prepared by treating a conventional 
gelatino--peptizer with a strong oxidizing agent, such as hydrogen 
peroxide. Tabular grain emulsions prepared in the presence of such 
peptizers are the subject of Maskasky U.S. Ser. No. 811,132 and 811,133, 
both filed Dec. 19, 1985, commonly assigned. These emulsions are 
particularly contemplated as host tabular grain emulsions for preparing 
emulsions according to this invention. 
It is also specifically contemplated to employ as host tabular grain 
emulsions for preparing emulsions according to this invention small, thin 
tabular grain emulsions, as disclosed by Daubendiek et al U.S. Ser. Nos. 
790,692 and 790,693, both filed Oct. 23, 1985, commonly assigned. The 
small, thin tabular grain emulsions are those having tabular grain mean 
diameters in the range of from 0.2 to 0.55 .mu.m, wherein the grains have 
average aspect ratios greater than 8:1 and account for greater than 50 
percent of the total grain projected areas. It is to be noted that a 0.2 
.mu.m diameter grain having an aspect ratio of 10:1 has a thickness of 
only 0.02 .mu.m. By forming peripheral ledges the average thickness of the 
grain can be further reduced. Since a procedure for preparing small, thin 
tabular grains has not yet been published, it is included to complete this 
disclosure in Appendix A, below. 
EXAMPLES 
This invention can be better appreciated by reference to the following 
specific examples: 
EXAMPLE 1 
A reaction vessel equipped with a stirrer was charged with 7.5 mmole of a 
freshly prepared (less than 3 hrs. old) 0.02 .mu.m AgBr emulsion 
containing 167 g/Ag mole deionized bone gelatin and made up to 32.5 g with 
water. To the emulsion at 40.degree. C. was added with stirring, 0.090 
mmole (6 mmole/Ag mole host) of the growth modifier 
5-bromo-4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene (GM-I) dissolved in 
water containing a small amount of triethylamine. To this mixture was 
added 15 mmole of a host tabular grain silver bromide emulsion (0.0033 
mole % AgI), of mean grain size 10.5 .mu.m, average tabular grain 
thickness 0.23 .mu.m, and average tabular grain aspect ratio 46:1. The 
tabular grains accounted for greater than 50 percent of the total grain 
projected area. The tabular grain emulsion contained about 17 g/Ag mole of 
bone gelatin and water to a total weight of 13.2 g. The pH was adjusted to 
6.0 at 40.degree. C. (all pH adjustments were with NaOH or HNO.sub.3, as 
required), and the pBr to 1.54 at 40.degree. C. with NaBr solution. The 
mixture was heated for 1 hr at 60.degree. C. 
FIG. 9 is a scanning electron micrograph of the resulting modified tabular 
grains, made with a 60.degree. angle of tilt. Greater than 50 percent of 
the total grain projected area was accounted for by tabular grains having 
ledges and the ledges accounted for greater than 5 percent of the the 
projected area of the tabular grains having ledges. 
EXAMPLE 2 
The host for Example 2 was a tabular grain pure AgBr emulsion, of mean 
grain size 4.8 .mu.m, mean tabular grain thickness 0.15 .mu.m, and average 
tabular grain aspect ratio 32:1. The tabular grains accounted for more 
than 50 percent of the total grain projected area. A fine grain emulsion 
provided for the Ostwald ripening procedure was a 0.02 .mu.m pure AgBr 
freshly made preparation. The procedure employed was like that for Example 
1, except that after the first 1/2 hour of ripening an additional 32.5 g 
(7.5 mmole) of the fine grain emulsion and an additional 0.090 mmole of 
GM-I were added. After the second addition the pH was adjusted to 5.83 at 
60.degree. C., and the pBr to 1.50 at 60.degree. C. The ripening was then 
continued at 60.degree. C. for the second 1/2 hour. 
FIG. 10 is a scanning electron micrograph of the resulting modified tabular 
grains, made with a 60.degree. angle of tilt. Greater than 50 percent of 
the total grain projected area was accounted for by tabular grains having 
ledges and the ledges accounted for greater than 5 percent of the the 
projected area of the tabular grains having ledges. 
EXAMPLE 3 
The host tabular grain emulsion for Example 3 was a tabular AgBrI (1 mole % 
I) emulsion of mean grain size 8.6 .mu.m, tabular grain thickness 0.140 
.mu.m, and average tabular grain aspect ratio 61:1. Tabular grains 
accounted for greater than 50 percent of the total grain projected area. 
The fine grain emulsion was a fresh remake of the emulsion used in Example 
1. The procedure was otherwise as described in Example 1. 
FIG. 11 is a scanning electron micrograph of the resulting modified tabular 
grains, made with a 60.degree. angle of tilt. Greater than 50 percent of 
the total grain projected area was accounted for by tabular grains having 
ledges and the ledges accounted for greater than 5 percent of the the 
projected area of the tabular grains having ledges. 
EXAMPLE 4 
The host for Example 4 was the same AgBrI (1 mole % I) emulsion as used in 
Example 3. The fine grain emulsion was a 0.02 .mu.m mean grain size AgBrI 
(1 mole % I) fresh preparation. The procedure was as described in Example 
1, except that Ostwald ripening was carried out for 1/2 hour. 
FIG. 12 is a scanning electron micrograph of the resulting modified tabular 
grains, made with a 60.degree. angle of tilt. Greater than 50 percent of 
the total grain projected area was accounted for by tabular grains having 
ledges and the ledges accounted for greater than 5 percent of the the 
projected area of the tabular grains having ledges. 
APPENDIX A 
Preparation of Small Thin High Aspect Ratio Tabular Grain Host Emulsions 
Emulsion A 
To a reaction vessel equipped with efficient stirring was added 3.0L of a 
solution containing 7.5 g of bone gelatin. The solution also contained 0.7 
mL of an antifoaming agent. The pH was adjusted to 1.94 at 35.degree. C. 
with H.sub.2 SO.sub.4 and the pAg to 9.53 by the addition of an aqueous 
potassium bromide solution. To the vessel was simultaneously added over a 
period of 12 s a 1.25M solution of AgNO.sub.3 and a 1.25M solution of 
KBr+KI (94:6 mole ratio) at a constant rate, consuming 0.02 moles Ag. The 
temperature was raised to 60.degree. C. (5.degree.C./3 min) and 66 g of 
bone gelatin in 400 mL of water was added. The pH was adjusted to 6.00 at 
60.degree. C. with NaOH, and the pAg to 8.88.degree. at 60.degree. C. with 
KBr. Using a constant flow rate, the precipitation was continued with the 
addition of a 0.4M AgNO.sub.3 solution over a period of 24.9 min. 
Concurrently at the same rate was added a 0.0121M suspension of an AgI 
emulsion (about 0.05 .mu.m grain size; 40 g/Ag mole bone gelatin). A 0.4M 
KBr solution was also simultaneously added at the rate required to 
maintain the pAg at 8.88 during the precipitation. The ABNO.sub.3 provided 
a total of 1.0 mole Ag in this step of the precipitation, with an 
additional 0.03 mole Ag being supplied by the AgI emulsion. The emulsion 
was coagulation washed by the procedure of Yutzy, et al., U.S. Pat. No. 
2,614,929. 
The equivalent circular diameter of the mean projected area of the grains 
as measured on scanning electron micrographs using a Zeiss MOP III Image 
Analyzer was found to be 0.5 .mu.m. The average thickness, by measurement 
of the micrographs, was found to be 0.038 .mu.m, resulting in an aspect 
ratio of approximately 13:1. Tabular grains accounted for greater than 70 
percent of the total grain projected area. 
Emulsion B 
Emulsion B was prepared similarly as Emulsion A, the principal difference 
being that the bone gelatin employed was prepared for use in the following 
manner: To 500 g of 12 percent deionized bone gelatin was added 0.6 g of 
30 percent H.sub.2 O.sub.2 in 10 mL of distilled water. The mixture was 
stirred for 16 hours at 40.degree. C., then cooled and stored for use. 
To a reaction vessel equipped with efficient stirring was added 3.0 L of a 
solution containing 7.5 g of bone gelatin. The solution also contained 0.7 
mL of an antifoaming agent. The pH was adjuated to 1.96 at 35.degree. C. 
with H.sub.2 SO.sub.4 and the pAg to 9.53 by addition of an aqueous 
solution of potassium bromide. To the vessel was simultaneously added over 
a period of 12 s a 1.25M solution of AgNO.sub.3 and a 1.25M solution of 
KBr+KI (94:6 mole ratio) at a constant rate, consuming 0.02 moles Ag. The 
temperature was raised to 60.degree. C. (5.degree.C./3 min) and 70 g of 
bone gelatin in 500 mL of water was added. The pH was adjusted to 6.00 at 
60.degree. C. with NaOH, and the pAg to 8.88 at 60.degree. C. with KBr. 
Using a constant flow rate, the precipitation was continued with the 
addition of a 1.2M AgNO.sub.3 solution over a period of 17 min. 
Concurrently at the same rate was added a 0.04M suspension of an AgI 
emulsion (about 0.05 .mu.m grain size; 40 g/Ag mole bone gelatin). A 1.2M 
KBr solution was also simultaneously added at the rate required to 
maintain the pAg at 8.88 during the precipitation. The AgNO.sub.3 provided 
a total of 0.68 mole Ag in this step of the precipitation, with an 
additional 0.02 mole Ag being supplied by the AgI emulslon. The emulsion 
was coagulation washed by the procedure of Yutzy, et al., U.S. Pat. No. 
2,614,929. 
The equivalent circular diameter of the mean projected area of the grains 
as measured on scanning electron micrographs using a Zeiss MOP III Image 
Analyzer was found to be 0.43 .mu.m. The average thickness, by measurement 
of the micrographs, was found to be 0.024 .mu.m, resulting in an aspect 
ratio of approximately 17:1. Tabular grains accounted for greater than 70 
percent of the total grain projected area. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.