P r o ceed in gs of t h e XXX V I Z ako p ane Sch oo l o f Ph ysi cs, Z ak opa n e 200 1
L ocal Stat es of T ell u r iu m i n Is olate d H elic al C h ain s
J . St anek
a, P. F o r na l
b, S.S. H af ne r
can d A . Pf it zne r
daI nst i t ut e of P h ysi cs, Jagi ell oni an Uni versi ty of Cra cow Reym onta 4, 30-059 Cra cow, Po l and
bInsti tute of Physi cs, Techni cal Uni versi ty of Cra cow, 30-084 Cra cow, Pol and
cCenter of Ma teri al s Sciences, Uni versi ty of Ma r burg , 35032 Ma rburg , G erm any
dDepartm ent of Ino rga ni c Chem i stry , Uni versi ty of Si egen, G erm any T h e l ocal elect ro nic state of T e in the helical chains of C uX Y2 ( 1À x )T e2 x
(X = C l, Br ; Y = Se, S) w ere stu died usin g the M ossbauer sp ectroscop y of 12 5T e at 10 K and 70 K . T he isolated chains of T e in these comp ounds are less rigid than those in elemental T e. T he determined data of quadrup ol e splittin g and Debye temp erature ÈD are indicati ve of strong lo cal ordering of T e in one of the tw o non -equivalent crystallogra phi c positio ns. I t w as not p ossible to resolve crystallogra phi c positio ns by the M ossbauer spectroscopy .
PACS numb ers: 76.80. + y
1. I n t r o d u ct io n
Pro p erti es of elementa l tel l uri um , T e, such as for exa mpl e pha se tra nsform a- ti ons fro m the hexag onal semi conducti ng pha se to the m onocl i nic m etal l ic pha se at hi gh pressures, ha ve been i nterpreted i n term s of the l o cal T e b ondi ng [1]. The crysta l structure of elementa l Te [2] consists of i nÙnite hel i cal chai ns whi ch i nter- act wi th each other. Ho wever, tha t stro ng i ntera cti on between the chai ns m akes i t ra ther di £ cult to esti m ate l ocal bondi ng wi thi n the chai ns. In the present pa- p er we shoul d l i ke to di scuss some resul ts obta i ned fro m sampl es of comp ositi on of Cu+ XÀ Y2 ( 1À x )T e2 x, where X = Cl , Br ; Y = Se, S wi th the use of1 2 5T e M ossbauer spectro scopy. In these com p ounds the i nÙni te T eY hel i cal chai ns are, sim i lar to el ementa l T e, the m ain structura l elements. They are well separa ted i n these cases, b eing hel d to gether by Cu+ XÀ Cu+ bri dg es. Thi s m ay al l ow m ore di rect insi ght i nto the di sti nct l ocal el ectro ni c T e conÙgura ti on whi ch m ust l ead to the form ati on of the chai ns. Further, an i nteresti ng aspect of Cu+ XÀ Y2 (1À x )T e2 x com p ounds may
(8 07)
b e the questi on of p ossibl e ato m ic orderi ng wi thi n the hel i cal chai ns o ver the T e, Y crysta l l ogra phi c p ositi ons, dep endi ng on the typ e of nearest nei ghb ours wi thi n the chai n or the typ e of connecti ng ani on X. For such study , M ossbauer spectro scopy
of 1 2 5T e m ay be an appro pri ate m etho d. X- ra y di ˜ra cti on studi es [3, 4] suggested
stro ng orderi ng of Te and Y over tw o non- equi val ent sites wi thi n the chai n.
2 . E x p er i m en t a l 2.1. Samples
The specim ens for thi s study were synthesi zed by m i xing CuCl , CuBr, T e, Se, S in appro pri ate pro porti ons and heati ng i n evacuated quartz capsul es at tem - p eratures o f ab out 300£C for few weeks [4].
Al l com p ounds b elong to the sam e structure typ e wi th the same structura l to p ol ogy. They crysta l l i ze i n the m onocl inic space group P 21= n. Pro j ecti ons of the crysta l structure are shown i n Fi gs. 1a and b. The chal cogen hel i cal chai ns are pa ra l l el to the b-axi s. They are hel d to gether by Cu { X{ Cu uni ts para l l el to the
c-axi s. There are two no n- equi val ent crysta l l ogra phi c positi ons for T e and Y :M 1
and M 2, wi thi n each chai n. These sites m ay b e di sti ng ui shed by thei r di stances to the ha l ide ani ons X or Cu. For the M 1 p ositi on the di stanceM 1{ X i s shorter tha n for M 2, whi l e for the M 2 positi on the M 2{ Cu di sta nce i s shorter tha n for
M 1. At Ùrst sight, S, Se, and Te may b e expected to enter b oth sites, M 1 as well asM . Ho wever, a previ ous study [3, 4] suggested tha t the T e ato ms pref erenti al l y o ccupy the M p ositi on.
For further di scussion three lo cal conÙgura ti ons of T e ato ms wi thi n one chai n wi l l be considered: conÙgura ti on˜ refers to Te wi th tw o other T e as nearest nei gh- b ours: { T e{ T e{ T e{ , conÙgura ti onÙrefers to Te wi th two Y as nearest nei ghb ours:
{ Y{ T e{ Y{ , and conÙgura ti on Û refers to T e wi th one T e and one Y as nearest
nei ghb ours: { Te{ T e{Y{ . It i s to b e anti cipa ted tha t for our consi derati on of such conÙgura ti ons, i t wi l l b e i rrel evant whether the center p ositi on T e, or the nei gh- b ori ng Te, is l ocated at M 1 or M 2 site.
2.2. M ossbauer spect roscopy of1 2 5Te
M ossbauer spectro scopy of1 2 5T e i s ba sed on resona nt a bsorpti on of 35.48 keV
Û ra di ati on whi ch results fro m the decay of the Ùrst exci ted nucl ear sta te,
I = + 3 = 2, to the gro und sta te, I = +1 = 2. The rel ati vely hi gh energ y of the tra n- siti on requi res usual l y m easurem ent at l ow tem pera tures. The m ain di sadva ntag e
of 1 2 5T e spectro scopy i s the rel ati vel y bro ad na tura l l i ne wi dth À = 5 :0 2 m m/s
at hal f m axi mum of the resonant absorpti on. Theref ore, ra ther \ p oor resol uti on "
i s expected. G eneral l y,À i s a few ti m es l arg er tha n the ra ng e of observed i som er shi fts, £. It i s about 3 ti mes sm al l er tha n the la rgest nucl ear qua drup ole spli tti ng,
 , rep orted so far. A comprehensi ve revi ew on 1 2 5T e M ossbauer spectro scopy i s presented i n [5]. In the fol l owing a few poi nts rel evant to the experi m ents rep orted here are describ ed.
As source m etastabl e1 2 5 mT e was used. A Mg31 2 4T eO6 m atri x was acti vated at the Ha hn{ Mei tner- Insti tut Berl i n f or 10 da ys i n a neutro n Ûux of 2È 10 neutro n= s cm . D ue to the hi g h Debye tem p erature, È , of Mg TeO [6], i t was p ossibl e to observe resona nt absorpti on wi th source hel d at ro om tem p era- ture. The absorbi ng sam pl e wa s m oun ted i n a gas-Ûow cryo sta t. For the detecti on of the 35.48 keV ra di ati o n, a germ ani um detecto r of hi ghest puri ty was used. Al - terna ti vely, a pro p orti onal counter coul d al so b e empl oyed, regi steri ng the \ escap e p eak" . The powdered absorb ers were pressed to pel l ets whi ch po ssesseda di ame- ter of 1.0 cm and conta i ned 20 m g of na tura l tel l uri um p er cm2 i n each case. The e˜ecti ve absorb er thi ckness, t , cal culated f or o ne absorpti on li ne, wast 0:5 at 10 K and t 0:2 at 70 K, obta i ned wi th the use of È as determ i ned (see secti on b elow). Thus, the appro xi mati on of thi n absorb er coul d b e appl i ed for the eval uati on of the data .
3. R esu l t s
Sp ectra of al l sam pl es were recorded at 10 K and 70 K. They exhi bi ted the typi cal nucl ear qua drup ol e-spl i t do ubl et as shown i n Fi g. 2. The hyp erÙne pa ra meters such as i som er shift £ ref erred to Mg T eO at 295 K, qua drup ole spl itti ng Â, l i ne wi dth À, and resona nt absorpti on areaA are col l ected i n T abl e.
The £ val ues were found to b e the sam e for al l the sam pl es wi thi n the exper- i m enta l erro r. For the CuCl Se2 (1 )T e2 series exhi bi ted a signi Ùcant decrease wi th i ncreasingx as shown i n Fi g . 3. For CuCl ST e i s the same as for CuCl SeTe whi l e for CuBrST e i s substanti al l y smal l er. It i s i nteresti ng to no te tha t the Â
val ues di d no t change b etween 1 0 K and 70 K. However,À showed a t 10 K as well as at 70 K the sam e typi cal bro adeni ng wi th respect to i ts na tura l val ue.
Fig. 2. M ossbauer spectra of 125 T e in CuC lSe2(1 À x )T e2 x. U pp er spectrum (x = 1) at 10 K (lef t) and at 70 K (right), low er spectrum (x = 0 : 2 5) at 10 K (lef t) and at 70 K (right). N ote the increased quadrup ol e splitting  w ith decreased f raction of T e. T he decreasing intensity w ith increasing temp erature is more pronounced f or the sample richer in T e.
Fig. 3. Dep endence of the quadrup ol e splitting  on x in C uC lSe2(1 À x )T e2 x. O pen circles ref er to the exp erimental data of T able. T he solid line is the dependence predicted assuming statistical distributi on of Se, T e over and . T he dashed line refers to complete ordering of T e into or .
T AB L E
125 T e M ossbauer data of Cu+XÀ Y2(1 À x )T e2 x, X = Cl, Br; Y = S, Se.T
| temp erature, £ | isomer shif t referred to Mg3T eO6 source at 293 K , Â | nuclear quadrup ole splitting, À | f ull width at half maximum of the resonant absorption, A | resonant absorption area, ÈD | Debye temp erature.
T £ Â À A ÈD
[K ] [ mm /s] [mm/s] [mm/s] [K ]
C uC lST e 70 1.7(2) 11. 2(2) 6.4( 5) 19. 8 132 10 1.7(1) 11. 4(1) 7.3(2) 49. 2
C uBrST e 70 1.7(1) 10. 0(2) 7.9(4) 23. 6 138 10 1.7(1) 10. 5(1) 8.4(2) 53. 5
C uC lSe2(1 À x )T e2 x
x = 1 70 1.7(1) 9.6(2) 6.2(3) 17. 4 138
10 1.7(1) 9.6(1) 6.9(2) 39. 6
0.8 70 1.6(1) 9.6(2) 6.9(4) 19. 4 138
10 1.6(1) 9.6(1) 6.9(2) 43. 6
0.5 70 1.7(1) 11. 6(1) 7.5(3) 24. 0 148
10 1.7(1) 11. 5(1) 7.2(2) 47. 8
0.25 70 1.8(1) 11. 1(2) 6.4(4) 23. 9 147
10 1.7(1) 11. 1(1) 6.6(2) 48. 1
Fig. 4. Dep endenc e of the ratio of 125 T e recoil- f ree f raction 70 K 10 K on the Debye temp erature ÈD used for determination of ÈD values plotted in Fig. 5.
The com pa ri son of the resonant absorpti on area, A, of the spectra mea- sured at two di ˜erent tem p eratures i n i denti cal geom etry al l owed determ i na ti o n of the Debye tem p erature ÈD for each sampl e. The cal cul ated rati o of
A ( T = 7 0 K)/ A ( T = 1 0 K) as functi on of ÈD for 1 2 5T e i s shown i n Fi g. 4.
Usi ng the experi m ental ra ti oA(7 0 K)/ A(1 0 K), ÈD was obta i ned for each sampl e.
The ÈD val ues are di splayed i n T abl e and Fi g. 5. In case of the CuCl Se2 (1À x )T e2 x
series an abrupt decrease i n ÈD b etween x = 0 : and x : was observed.
È
The i nvari able val ue of£ i n T abl e f or al l sam pl es is not surpri sing . The di ˜er- ence i n chem i cal bo ndi ng of T e i n Y conta i ni ng chai ns are, i n pri nci pl e, restri cted to sm all di ˜erences i n p shell p opul ati on, whi ch m ay onl y i ndi rectl y i nÛuence the el ectro n density at the T e nucl eus. Ho wever, i s expected to b e a qui te sensiti ve m easure of a ny ani sotro py of p shell .
4.1. Compar i son wi t h element al tel lur i um
The b ondi ng of Te i n two- coordi na ted T e i n T e b eari ng hel i cal chai ns i s predo mi na ntly determ i ned by px and py orbi ta l b ondi ng pa ra l l el to the shortest M{ M di sta nces i n the chai n. Howev er, the el ectri c Ùeld gradi ent whi ch determ i nes the nucl ear qua drup ol e spl itti ng results pri m ari ly fro m the lone pai r of el ectro ns i n ful l y occupi ed T e pz orbi ta l s pa ra l l el to the chai n axi s. It is i nteresti ng to note tha t  of elemental T e ( : m m /s at 4.8 K,  : m m/ s at 77 K)
i s signi Ùcantl y sma ll er tha n  = 9 : 6 m m/ s found i n CuCl T e2. Thi s smal l er Â
suggests som e addi ti onal Ùll i ng of the b ondi ng 5 px, 5 py orbi tal s a nd, theref o re, som e addi ti ona l b ondi ng b etween Te chai ns. Thi s conclusi on i s i n agreement wi th our value ÈD = 138 K for CuCl T e2, whi ch i s rem ark abl y l ower tha n tha t for el ementa l T e (ÈD = 180Ï 15 K [7]). The decrease i n  as rep orted for elemental T e b etween 4.8 K and 77 K ha s been i nterpreted as a result of to rsiona l m oti ons of the spira l chai ns. Such tem p erature dep endence wa s no t observed for our sam pl es.
Our results ra ther hi nt tha t to rsio n of i sol ated spira l chai ns hel d to gether by Cu+ ClÀ bri dges i s fro zen.
4.2. O r der i n g of T e i n CuC lSe2 (1À x )Te2 x
Our 1 2 5Te spectra of CuCl T e2 exhi bi ted one do ubl et onl y, wi tho ut nota bl e
l i ne bro adeni ng . D i sti nct 1 2 5T e hyp erÙne spl itti ng s at the i ndi vi dua l M and M
sites coul d no t be resol ved. Theref ore, i t was not p ossibl e to identi fy M and M
sites by 1 2 5Te resona nce. The geom etri cal di sti ncti on of M andM a pp ears to b e so smal l tha t any detecti on of intri nsi c qua drup ol e spli tti ngs canno t b e expected.
The observed e˜e ct i ve spl i tti ng s  ra ther result fro m nearest M and/ or M
o ccupanci es by Te or an Y ato m other tha n T e, e.g . Se.
Fi rst, l et us consider the hyp otheti cal case of complete order i ng for a pa rti c- ul ar chem i cal com po siti on wi thi n the system CuCl Se2 (1À x )Te2 x. The pro ba bi l i ty
p x for the cluster ˜ : { T e{ { Te{ wi thi n the chai ns i s
p˜ x for < x < : ; and (1 )
p˜ x x : for : < x < ; (2 )
for the cluster Ù { Se{ { Se{ , i t i s
pÙ x for < x < : ; and (3 )
pÙ x = x x 2 for : < x < ; (4 )
and Ùnal l y for the cluster Û { Se{ { Te{ , i t i s
pÛ x for < x < : ; and (5 )
pÛ x = x x x for : < x < : (6 )
These pro ba bi l i ti es are di splayed i n Fi g. 6 (l eft). Acco rdi ngl y, the respecti ve pro b- abi l i ti esp for compl ete di sorder i ng, i .e. sta ti sti cal di stri buti on of Te o ver al l M
and M sites, are
p˜ x x2; (7 )
pÙ x x 2; (8 )
pÛ x x x ; (9 )
as shown i n Fi g. 6 (ri ght).
Fig. 6. Probabil iti es p of the clusters ˜ , Ù, and Û on telluri um concentration, x, f or completely ordered phases (lef t) and completely disordered phases (right).
Acco rdi ng to thi s simpl e m odel , the observed values ( x ) i n a bi na ry sys- tem CuXY 2 (1À x )Te2 x represent wei ghted avera ges of ˜ ,  Ù, and ÂÛ, i.e. the i ndi vi dua l qua drup ol e spli tti ngs f or the clusters ˜ ,Ù, and Û, respecti vel y
 ( x p˜ x ˜ pÙ x ÂÙ pÛ x ÂÛ: (1 0)
The val ue of ˜ i s kno wn fro m the CuCl Te2 spectrum : ˜ : m m/s (T abl e).
Tha t of ÂÛ m ay b e assumed general l y asÂÛ : Â ˜ Â Ù . Fro m (1 0) Â Ù
m ay b e obta ined using , i n pri nci pl e, j ust one value of determ ined exp eri m ental l y fro m a sam pl e of arbi tra ry comp ositi onx:
 ٠m m=s  : p˜ pÛ = pÙ pÛ : (1 1)
In case of compl ete order, i t is readi l y seen, wi thi n the appro xi ma tio n o f thi s mo del, tha t Eq. (1 1) i s reduced by i nserti on of Eqs. (1 ), (3 ), and (5) to ÂÙ Â x for any comp ositi on wi th x < : .
W e may no w compa re the exp eri menta l qua drup ol e spli tti ngs of the sys- tem CuCl Se2 (1À x )T e2 x repo rted i n Tabl e wi th  deri ved f rom o ur sim pl e m odel for com pl etel y ordered pha ses as wel l as tho se for com pl etely di sordered ones by i nserti on of the appro pri a te pro babi li ty term s p deÙned i n Eqs. (1 ) to (9 ) i nto Eqs. (1 0) and (1 1). Fi rst, the com pl etely ordered pha ses wi l l be consi dered. In- serti ng the experi m ental  da ta f orx : andx : i nto (1 1) and using the
p val ues of Eqs. (1), (3 ), and (5) resul ts i n the mo del val uesÂ0 : 2 5 : m m/s
and  0 : 5 : m m/ s, respecti vely. Inserti ng the experi m enta l  for x :
i nto (1 1) and p of Eqs. (2 ), (4 ), and (6 ) accordi ngl y results i n the mo del val ue 9.6 m m/ s.
 x for the compl etely di sordered pha ses m ay b e cal culated wi th the use of Eqs. (7 ), (8 ), (9 ), and (10):
 x m m=s : x2 ÂÙ x 2 :  ٠x x ; or (1 2)
 ٠ : x = x : (1 3)
Equa ti on (1 3) i s val i d for the enti re ra ng e of com p ositi onsx < x < ).
The sam e pro cedure was appl ied for the experi m ental ly determ i ned val ues
ÈD( x ). The predi cted, resul ti ng dep endence of ÈD on the concentra ti on x i s i l - l ustra ted i n Fi g. 5. The pri ncipa l sim i l arit y of Fi gs. 3 a nd 5 shoul d b e no ted i n vi ew of the no n- correl ated determ i nati on of  and resona nt absorpti on areas A
(T abl e). Com pari son of m easured and predi cted ÈD( x ) as well as the  ( x ) values, ba sed on our simpl e m odel of l ocal clusteri ng, favo urs stro ngl y lo cal orderi ng of T e o ver one of the M sites, whi ch may b eM 1 or M 2. As m enti oned at the b egi nni ng of thi s section, i t i s not p ossibl e to di sti ng ui sh between M 1 and M fro m T e spectro scopy al one.
4.3. T he rol e of chalcogen and hal i de ani ons
The substi tuti on of Se by S do es no t chang e the electro ni c state of T e sig- ni Ùcantl y as concl uded fro m the T e spectra of CuCl STe. Howev er, further re- pl acement of Cl by Br (T abl e) reduces the val ue of Â. Thi s is i ndi cati ve of some addi ti ona l electro n tra nsf er fro m Br to the p , p orbi tal s o f T e. The Y{ X di s- ta nce app ears to b e shorter at the M site tha n at theM site. Thus, the i nÛuence of Br substi tuti on for Cl on the electro ni c T e sta te, i ndeed, suggests tha t T e i s or- dered i nto the M p ositi on as i t was concluded fro m X- ra y di ˜ra cti on.
W e tha nk Doro thea Al b er, Ha hn{ Mei tner- Insti tut Berl i n for acti vati ng the T e sources. Thi s work was pa rtl y Ùnanced by the pro j ect W T Z POL 9 8-031 of D LR (BMBF), G erma ny.
