Crystalline Bi.sub.2 Ge.sub.3 O.sub.9 and its preparation

A monocrystalline Bi.sub.2 Ge.sub.3 O.sub.9 is formed, for example, via the Czochralski technique from a melt containing pure Bi.sub.2 O.sub.3 and GeO.sub.2 at a molecular ratio of 1:3. This crystal is useful as an x-ray spectrometer crystal and/or as a light defector crystal in conjunction with an ultrasonic deflection field properly applied to such crystals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to bismuth-germanium-oxide materials and somewhat 
more particularly to such a material with the formula Bi.sub.2 Ge.sub.3 
O.sub.9 in crystalline form useful in various devices, such as an x-ray 
spectrometer crystal or as a photo-acoustic deflector crystal. 
2. Prior Art 
Russian Journal of Inorganic Chemistry, Vol. 9, No. 2, pages 226-230 (1964) 
describes the bismuth oxide-germanium-dioxide system and indicates that 
the phase diagram of Bi.sub.2 O-GeO.sub.2 has stable compounds only for 
two mixture ratios, namely, at Bi.sub.2 O.sub.3 :GeO.sub.2 ratio of 2:3 
and at Bi.sub.2 O.sub.3 :GeO.sub.2 ratio of 7:1. 
Journal of Research of the National Bureau of Standards-A. Physics and 
Chemistry, Vol. 68A, No. 2, pages 197-206 (1964) corrects the Bi.sub.2 
O.sub.3 :GeO.sub.2 ratio from 7:1 to 6:1. As to a Bi.sub.2 O.sub.3 
:GeO.sub.2 ratio of 1:3, which is relevant to the invention, the prior art 
established that at such a ratio, only eutectic materials exist. Eutectic 
materials of this type are inevitably multiphased so that monocrystalline 
structures from eutectic materials are not possible. 
SUMMARY OF THE INVENTION 
The invention provides a new crystalline material which has not previously 
been produced or available to the art in any manner. This new crystalline 
material is technically useful for solving new technical problems or at 
least providing fundamental improvements to known technical solutions. One 
such new material consists of a bismuth-germanium-oxide material having an 
overall formula Bi.sub.2 Ge.sub.3 O.sub.9 and occurring as a stable 
single-phase chemical compound existing in crystalline and monocrystalline 
form. 
A process for producing such new crystalline material comprises 
establishing a melt composed of relatively pure Bi.sub.2 O.sub.3 and 
GeO.sub.2 at a ratio of 1:3, contacting such melt with a seed crystal and 
controllably drawing or pulling the seed crystal from the melt so that a 
monocrystal forms thereon, in accordance with the Czochralski technique. 
The Bi.sub.2 Ge.sub.3 O.sub.9 crystals of the invention are useful as x-ray 
spectrometer crystals or as light deflectors when an ultrasonic deflection 
field, which forms an optical lattice, is established in such crystals.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In direct contrast to prior art teachings relative to bismuth 
oxide-germanium oxide systems, the invention provides a totally unexpected 
genuine chemical compound producible in crystalline and monocrystalline 
form. 
The chemical compound and/or monocrystal of the invention exhibits 
particularly favorable characteristic values for use in x-ray 
spectrometry, preferably with soft x-rays and for use in technical 
procedures based on photo-acoustic effects. 
The inventive crystals are produced via a crystal drawing process from a 
suitable melt contained in a crucible, which is fundamentally known per 
se. 
In such crystal production process, which corresponds to the so-called 
Czochralski technique, very pure bismuth trioxide (99.995% pure) and very 
pure germainum dioxide (99.999% pure) are admixed in a molecular Bi.sub.2 
O.sub.3 :GeO.sub.2 ratio of 1:3, positioned in, for example, a platinum 
crucible having, for example, a diameter of 40 mm and a height of 40 mm 
and are heated to about 1275.degree. K. so as to form a uniform melt. A 
seed crystal is controllably brought into contact with this melt and 
rotated, in an oxygen or air atmosphere so as to avoid reduction of the 
molten oxides and slowly withdrawn from the melt, which is maintained at 
about 1263.degree. to 1273.degree. K. so that a monocrystal forms on the 
seed. The so-formed monocrystal has a composition precisely corresponding 
to the composition of the melt. 
As exemplary process parameters, a seed crystal rotation of 100 revolutions 
per minute and a crystal drawing rate of 5 mm per hour produces 
monocrystals of the invention having a length of 50 mm and a diameter of 
15 mm which are entirely free of occlusions or other defects. If too high 
of a drawing speed is utilized, microscopic occlusions of platinum and gas 
bubbles occur in the formed crystals and are thus to be avoided. 
A Bi.sub.2 Ge.sub.3 O.sub.9 monocrystal of the invention is useful as an 
x-ray spectrometer crystal. As will be appreciated, a multi-phase eutectic 
material mixture could not be utilized as a spectrometer crystal. A number 
of spectrometer crystals are known and are used in various systems. For 
example, potassium hydrogen phthalate with a double lattice spacing (2d) 
of 26.632A is known, as are thallium hydrogen phthalate with a 2d spacing 
of 25.900A, .alpha.-quartz with a 2d-spacing of 2.749A and topaz with a 
2d-spacing of 2.712A. The monocrystals of the invention have a 2d lattice 
spacing (corresponding to 2a.sub.2) of 19.572A for the [001]-direction, 
i.e., parallel to the six-number axis system. Particularly for use with 
average to soft x-rays, crystals of the invention are substantially 
superior to .alpha.-quartz and/or topaz crystals because of its higher 
coefficient of diffusion. In comparison to known phthalate crystals, 
monocrystals of the invention have extremely important advantageous 
properties, namely they are water insoluble, exhibit high chemical 
stability and extremely high mechanical stability. In comparison to all 
known spectrometer crystals, the Bi.sub.2 Ge.sub.3 O.sub.9 crystals of the 
invention, in the [001] direction, have an extremely low thermal 
coefficient of expansion, which renders superfluous thermostabilization of 
the spectrometer crystal over a wide temperature range. The crystals of 
the invention also exhibt a very desirably high absorption coefficient for 
x-rays. 
Monocrystals of the invention can be split extremely easily, for example in 
comparison to mica, at right angles to the [001] direction, i.e., parallel 
to the (001) plane. This results in a considerable reduction in production 
costs of finely processed Bi.sub.2 Ge.sub.3 O.sub.9 monocrystals of the 
invention. It must be remembered that natural split surfaces of crystals 
generally have better optical qualities than crystal surfaces artificially 
produced, even with high outlays, as by sawing, grinding, lapping or 
polishing. 
When a Bi.sub.2 Ge.sub.3 O.sub.9 crystal of the invention is used to 
diffract light in a Debye ultrasonic field, a high degree of diffraction, 
i.e., beam deflection, can be achieved because of the very pronounced 
photo-elastic effect of this crystalline material. The inventive crystals 
of the invention have only a very limited self-absorption over the wave 
length range extending from about 0.290 .mu.m to a few .mu.m. 
Consequently, crystals of the invention, because of this property are 
useful in systems using laser radiation. 
For example, lead molybdate presently used in a laser high speed printer 
system, known per se, can be advantageously replaced with a crystal of the 
invention. In a printer system of this type, the crystal functions to 
deflect the laser beam used for printing to its proper position at 
extremely high rates of speed. 
Referring now to FIG. 1, a platinum crucible 1 is positioned in an operable 
heating means 2 capable of controllably heating the crucible and its 
contents to about 1300.degree. K. A measured amount of pure Bi.sub.2 
O.sub.3 (99.995% pure) and pure GeO.sub.2 (99.999% pure) so as to provided 
a Bi.sub.2 O.sub.3 :GeO.sub.2 ratio of 1:3 is placed in the energized 
crucible so as to form a melt 4. A seed crystal 8 is mounted at the end of 
a rotatable shaft 5 which is driven by rotating means 6. The shaft is 
lowered into contact with the melt and slowly withdrawn at a given 
longitudinal rate in the direction 7 while rotating so that a monocrystal 
3 composed of Bi.sub.2 Ge.sub.3 O.sub.9 is drawn out of the melt, as 
shown. The crystal 3 rotates together with the shaft 5 and the seed 
crystal 8. 
FIG. 2 illustrates an exemplary x-ray spectrometer means, such as an 
operative x-ray goniometer system 11, operative principles and details of 
which are known, for example, see German Offenlegungsschrift No. 26 37 
945. Essentially, such a system comprises an operative x-ray source 12, a 
wafer-like Bi.sub.2 Ge.sub.3 O.sub.9 monocrystal 13 of the invention and a 
detector suspender 15, which is provided with a slit diaphragm 14. The 
crystal 13 is positioned in a suitable support (not shown) in which the 
crystal can be selectively rotated or adjusted about an axis which is at 
right angles to the plane of FIG. 2 in angular positions .theta., relative 
to the x-ray source 12. As is conventional in a goniometer, the detector 
15 is moved into an angular position 2.theta.. The crystal 13 functions as 
an x-ray spectrometer crystal in this and similar devices. 
FIG. 3 illustrates an exemplary light deflector system wherein a Bi.sub.2 
Ge.sub.3 O.sub.9 monocrystal of the invention is utilized as a light 
deflector functioning in accordance with the Debye-Sears effect. A light 
radiation or beam 23, which may be a laser beam, emerges from a light 
source 21 and impinges on crystal 22. Within the crystal 22, the beam is 
diffracted by a vertical ultrasonic wave field 26 and emerges as beam 24 
extending at an angle to the original beam 23. The beam 24, which is 
deflectable in a controlled manner can be used, for example, to print a 
photo-sensitive paper 25. The ultrasonic wave field 26 can be produced, 
for example, via a piezo-electric body 26 applied onto the crystal 22. A 
generator means 28 is operationally coupled to the piezo body 27 so that a 
controllable voltage is fed to the body 27 so as to produce a controllably 
variable electric field, which, because of the piezo effect, causes 
corresponding mechanical contractions and expansions in the piezo-body 27. 
These movements in body 27 are transferred to crystal 22 wherein, when the 
size dimensions are appropriate, they produce a vertical (in the 
embodiment shown) ultrasonic wave field which acts as an optical lattice. 
As is apparent from the foregoing specification, the present invention is 
susceptible of being embodied with various alterations and modifications 
which may differ particularly from those that have been described in the 
preceding specification and description. For this reason, it is to be 
fully understood that all of the foregoing is intended to be merely 
illustrative and is not to be construed or interpreted as being 
restrictive or otherwise limiting of the present invention, excepting as 
it is set forth and defined in the hereto-appended claims.