RIKEN Accel. Prog. Rep. 48 (2015)

Ⅲ-1. Atomic & Solid State Physics (Ion)

-V22H
H††
Lattice
Lattice location
location andand density
density distribution
distribution ofof hydrogenhydrogen inin bb11-V*1,*2E.
E.Yagi,Yagi,*1,*2T.
T.Hayashi,Hayashi,*2*2T.
T.Yoshida,
Yoshida,*2*2S.
S.Koike,Koike,*3*3N.
N.Higami,Higami,*2*2K.Hirabayashi,
K.Hirabayashi,*2*2A.
A.Takebayashi,
Takebayashi,*2*2H.
H.Matsuba,
Matsuba,*2*2andand*1*1
K.
K.Ogiwara
Ogiwara

OnOn thethe atomistic
atomistic state
state ofof hydrogenhydrogen inin VV22H,
H, itit has
has beenbeenreportedreported thatthat hydrogenhydrogen atoms
atoms areare distributed
distributed randomly
randomly atat
tetrahedral
tetrahedral (T)(T) sites
sites inin thethe aa-phase-phase with
with aa bcc
bcc crystal
crystalstructure
structure atathighhigh temperaturestemperatures above
above about
about 470
470 K,
K, whereas
whereasinin thethe low-temperature
low-temperature bb11-phase-phase below
below about
about 450450 K,
K, theythey
occupyoccupy specificspecific octahedral
octahedral (O(Ozz)) sites
sites with
with anan ordered
orderedarrangement.
arrangement.1)1) TheThe bb-V
H has
has two
two different
different crystal
crystal-V22H
structures,
structures, aa body-centred
body-centred tetragonal
tetragonal (bct)(bct) structure,
structure, andand aa
body-centred
body-centred monoclinic
monoclinic one
one (bcm)(bcm) with
with slightslight inclination
inclination
ofof thethe c-axis
c-axis(( aa00 ≈≈90
90.3.3--9191  )) (a(a pseudo-tetragonalpseudo-tetragonal
1)1)structure),structure), both
both ofof which
which have
have anan axial
axial ratio
ratio between
betweena-anda-and c-axes
c-axes cc00 aa00 ≈≈11.1.1, , depending
depending onon thethe condition
condition forforcrystalcrystal growth.
growth.1)1) When
When tensile
tensile stressstress isis applied
applied along
along thethe[001][001] axis
axis during
during transformationtransformation fromfrom thethe aa toto bb11-phases,-phases,
thethe VV22HH crystallizes
crystallizes into
into thethe bct
bct structure,structure, while
while without
without
tensiletensile stressstress itit crystallizes
crystallizes into
into thethe bcm
bcm structure.
structure. The
The OOzz
sites
sitesareareoctahedral
octahedral(O)(O)sitessitesbetween
betweentwo
twoadjacentadjacentVVatoms
atoms
alignedaligned along
along thethe c-axis
c-axis (z-axis)(z-axis) (Fig.1).
(Fig.1). The
The changechange ofofhydrogenhydrogen site
site fromfrom TT toto OOzz sites
sites implies
implies thatthat thethe OOzz-site-site
occupancy
occupancyisis stabilizedstabilized bybyelongation
elongation ofof thethe c-axis,
c-axis, which,which,
conversely,
conversely,has
hasbeenbeeninduced
inducedby
byforces
forcesexerted
exertedby
byhydrogenhydrogenatoms
atoms onon neighbouringneighbouring VV atoms.
atoms. The
The mechanism
mechanism ofofoccurrence
occurrenceof
ofthe
thebctbctstructure
structureandandthetheOOzz-site-siteoccupancy
occupancyininthethe bb11-phase-phase waswas theoreticallytheoretically studiedstudied forfor thethe uniform
uniform
elongation
elongation ofof thethe c-axis
c-axis under
under thethe condition
condition ofof constantconstant
volume,volume, andand itit waswas proposed
proposed thatthat thethe elongation
elongation ofof thethe))c-axisc-axisisiseffective
effectivefor
for thetheOOzz-site-siteoccupancy.
occupancy.22 Therefore,
Therefore, toto
examineexamine this
this effectiveness,effectiveness, inin thethe present
present study,
study, thethe site
site
occupancy
occupancy ofof hydrogenhydrogen inin both
both tetragonal
tetragonal bb-V
H andand-V22H
monoclinic
monoclinicbb-V
Hisistotobe
beinvestigatedinvestigatedinindetail.
detail.
-V22H
Another
Anotherfundamental
fundamental problem
problemon
onhydrogenhydrogenisisthethedensity
density
distribution
distribution ofof hydrogenhydrogen aroundaround its
its occupation
occupation site.
site. ThisThis
problemproblem isis related
related toto thethe extent
extent ofof wave
wave function
function ofofhydrogen.
hydrogen. Experimental
Experimental values
values hitherto
hitherto reported
reported byby aa
neutronneutron diffraction
diffraction method
method forfor thethe tetragonal
tetragonal bb-V
H areare-V22Hlargerlargerthan
thanthethetheoreticallytheoreticallycalculated
calculatedvalues
valuesand,and,therefore,
therefore,
3,4)
there
therehas
hasbeenbeenaalarge
largediscrepancy
discrepancybetween
betweenthem.
them.3,4)
InIn order
order toto clarify
clarify thethe sitesite occupancy
occupancy andand thethe spatial
spatialdensitydensitydistribution
distribution ofofhydrogen,hydrogen, thethe channelling
channellingmethod
method isis
veryvery useful.
useful. InIn thethe present
present study,
study, thethe channelling
channelling method
methodutilizing
utilizingaanuclear
nuclearreaction
reaction11H(H(1111B,B,aa))aa
aawithwithaa1111BBbeam
beamofofan
an energy
energy ofof about
about 22 MeV,
MeV, which
which had
had been
been developed
developed toto5,6)
locatelocatehydrogenhydrogendissolved
dissolvedin
inTa
TaandandV,V,5,6)
isisapplied.
applied.
TheThe lattice
lattice location
location ofof hydrogenhydrogen andand its
its density
density
distribution
distributionin
inthe
thetetragonal
tetragonalbb11-V-V22H
Hsinglesinglecrystal
crystal††Condensed
Condensedfrom
fromthethearticle
articleininJ.J.Phys.
Phys.Soc.
Soc.Jpn.
Jpn.81,
81,044602
044602(2012)(2012)

＊1
＊1

RIKEN
RIKENNishina
NishinaCentre
Centre＊2＊2SchoolSchoolofofScience
ScienceandandEngineering,Engineering,Waseda
WasedaUniversity
University
Department
DepartmentofofPhysics
PhysicsII,
II,TokyoTokyoUniversity
UniversityofofScience
Science＊3＊3preparedprepared under
under tensile
tensile stressstress andand thethe monoclinic
monoclinic
bb11-V-V22H
H1.1
single crystal
crystal preparedprepared without
without tensile
tensile1.1 single
stressstress have
have been
been studiedstudied atat room
room temperature.
temperature. TheThefollowing
following results
results have
have beenbeen obtained.
obtained. InIn these
these crystals,
crystals, HH
atoms
atoms areare locatedlocated atat octahedral
octahedral (O)(O) sites
sites withwith different
different
distributions
distributions over
over OOxx, , OOyy, , andand OOzzsites.
sites. In
In thethe tetragonal
tetragonal
bb11-V-V22H,
H, about
about 80%
80% ofof thethe HH atoms
atoms areare locatedlocated atat OOzzsites,
sites,
about
about10%
10%of
ofthemthematatOOyysites
sitesandandaboutabout10%
10%ofofthemthematatOOxxsites.
sites. In
In thethe monoclinic
monoclinicbb11-V-V22HH1.1
about 65%
65% ofof thethe HH
1.1, , about
atoms
atomsarearelocatedlocatedatatOOzzsites,
sites,about
about 20%
20%of
ofthemthematatOOyysites
sites
andandabout
about15%
15%of
ofthemthematatOOxxsites.
sites.These
Theseresults
resultssupport
supportthe
thetheoreticaltheoretical prediction
prediction thatthat hydrogenhydrogen preferentially
preferentiallyoccupies
occupies
OOzzsites
sitesunder
undertensile
tensilestressstressalong
alongthethez-axis
z-axis(c-axis).
(c-axis).
InIn both
both crystals,
crystals, thethe HWHM
HWHM ofof thethe density
densitydistribution
distributionofofhydrogenhydrogenisisabout
about 0.35
0.35 ÅÅininthe
the[100][100]direction,
direction,whereas
whereasitit isis lessless than
than 0.13
0.13 ÅÅ inin thethe [001][001] direction
direction on
on thetheassumption
assumptionof
ofthe
theGaussian
Gaussiandistribution.
distribution.TheTheHWHM
HWHMininthe
the[110][110] direction
direction has
has been
been measured
measured only
only inin thethe tetragonal
tetragonal
crystalcrystal to
to be
be about
about 0.35
0.35 Å.
Å. These
These values
values areare muchmuch smaller
smallerthanthan those
those hitherto
hitherto obtained
obtained byby thethe neutron
neutron diffraction
diffraction
method,method, andand close
close totothe
the values
values theoreticallytheoreticallycalculated
calculated forforthethegroundgroundstate
state(~0.28(~0.28ÅÅfor
forthethe[100][100]direction
directionand
and~0.093~0.093ÅÅforforthethe[001][001]direction).
direction).

Fig.1.
Fig.1.Two
Twokinds
kindsofofTTsites,
sites, TT11andandTT22, ,andandthree
threekinds
kindsofofOO
sites,sites, OOxx. . OOyy andand OOz,z, inin thethe bct
bct structure.
structure. ForFor each
each type
type ofofsite,
site,only
onlysome
someofofequivalentequivalentsites
sitesareareindicated.
indicated.
References
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