The thermal evolution of the hyperfine quadrupole interaction in (NH4)2ZrF6 has been investigated via TDPAC from 14 to 400 K. A phase transition is detected near 370 K. The low-temperature phase exhibits two sites equally populated for181Ta probes. The associated electric field gradients are described by both high asymmetry parameters and high interaction frequencies. On the other hand, the high-temperature phase corresponds to an unique axially symmetric and more intense electric field gradient.Facultad de Ciencias Exacta

Caracoche, María Cristina

López García, Alberto Raúl

Martínez, Jorge Alberto

Rivas, Patricia Claudia

Rodríguez, Agustín Mario

Hyperfine Interactions

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Hyperfine Interactions 14(1983)227-231 227 TEMPERATURE DEPENDENCE OF THE HYPERFINE INTERACTION IN (NH,)2ZrF 6 A.M. RODRIGUEZ*, J.A. MARTINEZ**, M.C. CARACOCHE**, P.C. RIVAS and A.R. LOPEZ GARCIA*** Depart~nento de Fisica, Facultad de Ciencias Exaotas, Universidad National de La Plata, CC 67, 1900 La Plata, Argentina Received 20 June 1983 The thermal evolution of the hyperfine quadrupole interaction in (NH~)2ZrF 6 has been investigated via TDPAC from 14 to 400 K. A phase transition is detected near 370 K. The low-temperature phase exhibits two sites equally populated for 181Ta probes. The associated electric field gradients are described by both high asymmetry parameters and high interaction frequencies. On the other hand, the high-temperature phase corresponds to an unique axially symmetric and more intense electric field gradient. I .  In t roduct ion Continuing our high-resolution TDPAC measurements on hafnium and zirconium complexes [I-3], we report in this paper results on the thermal evolution of the quadrupole hyperfine interaction in (NH4)2ZrF 6 from 14 to 400 K. 9 The easy substitution of Zr atoms by Hf atoms and the well-known properties of the 482 keV (~+) level of laZTa produced by 8- decay of Z81Hf make the application of the TDPAC technique suitable for an in- vestigation of hyperfine interactions in zirconium compounds. No previous literature on hyperfine data has been found on the complex under investigation. Bulk information reported by Gaudreau [4] indicates that (NH~)2ZrF 6, when produced via thermolysis of (NH4)3ZrF~, decomposes into (NH~)ZrF S around 393-398 K. Two phases are reported by this author for (NH~)2ZrF 6, depending on thermal treatments: the dis- ordered phase when (NH~)3ZrF ~ is heated to 473 K and quenched, and the ordered phase when a slower cooling is performed. Their X-ray spectra are quite similar, differing only in weak lines. 2. Experimental The compound was prepared by dissolving zirconium (10% Hf) in con- centrated hydrofluoric acid and adding an aqueous solution of ammonium fluoride in stoichiometric proportions. Crystals were finally obtained after slow evaporation at room temperature. X-ray analysis on a powder sample showed that, with the exception of one weak line, the diffrac- tion pattern was consistent with the ordered phase of (NH~)2ZrF 6. It was also possible to sort out lines belonging to (NH4)3ZrF ~. *Fellow of CICPBA, Buenos Aires, Argentina. **Member of Carrera del Investigador Cient[fico, CICPBA, Buenos Aires, t~gentina. ***Member of Carrera del Investigador Cientifico, CONICET, Argentina. 9 J.C. Baltzer Scientific Publishing Company 228 A.M.  :,h>,/zl~Tu, ;: ,.~ ! / . ,  l l I ' l  l;I IN f ix , )  SrE) T h e  l~] l{f  a c t i v i t y  w a s  o b t a i n e d  b,/ i r r a d i a t i n q  t h e  c o m p o u n d  i n  t h e  RA-3 reactor of Comision Nacional de Enerc~a At~Smica, Arqentina. The high-resolution TDPAC setup (2~ : 0.75 ns) and turther data handling, as well as the fittin<7 proccduTe, have already b'ecn de- scribed in ref. [ ] ] . A linear combination of static electric quadrupole interaction perturbation factors of the form (] (t) = qL 0 (8) + ~ c. n (rj)e =$ ' ntcos (,. i;t) n I led to satisfactory fits for all temperatures. The coefficients of the linear combination were inter! reted as the relative populations of the sites occupied by !~iTa probes at different environments. As it is known, TDPAC t<.chnlquo allows the determination of, from ~'n values, the quadrupo]e interaction frequent-" ~Q (proportional to the maximum component \'z;~ of the electric field <~radient (EFG)) and the asymmetry parameter R defined as n (V• A lorentzian distribution of width ~ was introduced to account for a possible spread of the interaction frequencies. The uncertainty in the temperature determination was about + K. 3. Resul ts and d iscuss ion In an attempt to estimate the influence of (NIl.) <ZrF 7 present n the sample, and bearing in mind that its decomposition temperat[:re into {NH ) ',{rF was reported to be approximately 395 K, results at room temperature were compared before and after a thermal treatment at 400 K. No significant chanqes of the interaction were observed, unless near t = 0. So, the influence of (N~{..) ZrF: was disregarded and the fits were performed nealecting the first three channels {t < I ns). Fig.] shows the spin-rotation curves obtained at different tem- peratures. In fig.2 the fitted quadrupole hyperfine parameters, tabu- fated in table ], are plotted versus temperature. ;',11 interactions were found to have ne<Jligible frequency distribution widths (6<21) which, thus, have not been included in the figure. At low temperature, two interactions were needed to e::plain the observed perturbation. Boti] interactions exhibit ti~e same population, but the thermal behaviours of the corresponding EFGs are rather dif- ferent. Indeed, one of them seems to be not very dependent on tempera- ture: while the interaction frequency weakly decreases, the associated rl value shows a sligl:t variation presenting a maximum at around 170 K. The other one shows a stronger thermal dependence. As can be observed in fig.2, the quadrupole frequency of this site decreases and the )1 value monotonically increases, reaching the full asymmetry (~-I) near 343 K. Hereinafter, a new intense and symmetric EFG must be included to explain part of the observed interaction. Its population grows with temperature at the expense of the former ones. This undoubtedly indi- cates that a phase transition is taking place. At 400 K, where the relative population of the new phase attains 80%, an additional contribution to the EFG is needed to give account of the experimental data. This situation was interpreted in terms of thermolysis of the compound, reported by Gaudreau [4] at the same temperature. A more detailed investigation of this decomposition, as well as the TDPAC study of the products involved, is in progress. Unfortunately, crystalline structure data at any temperature are lacking and no theoretical predictions of EFG at iSiTa sites could be performed. Nevertheless, the mere inspection of the thermal behavi- o & o C) ~-,f ~,, 0 o o 0 r~ t~ F.s L 230 A.M. Rodriguez et al., HFI in (NH~):ZrF~ 4 fC/o) % I . 1201 ,, 1'0 I0,8 0 6  , 6,4 0 A A ~ 9 i i i A A , A v 9 9 v 9 9 160 260 360 T(K) 400 Fig.2. Thermal evolution of the fitted hyperfine parameters. Different syn~ols are used to show the evolution of each interaction. At 400 K, parameters corresponding to a 10% fraction of a fourth contribution to the EFG have not been plottt~d. Table 1 Quadrupole interaction parameters deduced from the fits at different telnperatures T(K) fl(<) ~Ql(Mrad/s-i ~1 f2(%) WQ2(Mrad/s-I n 2 f~(%) ~Q~(Mrad/s -1 14 52u 1461 0.521 45~ 1191 0.55 l 49 523 1481 0.501 453 1201 0.53 l I01 433 1451 0.531 543 121~ 0.611 152 48~ 1421 8.57 ! 49: 123 ! 0.651 200 505 1381 0.641 485 1241 0.651 251 52~ 1321 0.73 I 484 125 I 0.591 293 46. 1241 0.841 533 1261 0.531 320 48~ 1191 0.901 52: 1251 0.481 343 48,, 1151 1.00" 42 1261 0.451 360 324 1142 0.998 34~ 1251 0.431 388 185 1161 0.881 22~ 127 I 0.391 400 13~ 1141 0.86 i0~ 1291 0.36~ I0 S 203 l 34 e 187~ 60~ 186 l 677 1871 The third component of the EFG from 343 to 400 K was observed to be axially sym- metric. *This parameter was kept fixed. A.N. Rodrlguez et at., IIFI in (NII~)2ZrF c 231 our of the hyperfine interaction from 14 to 400 K allows us to point out some interesting features. First of all, a phase transition is detected around 370 K. Due to the wide temperature range within which the transition takes place, the above temperature was stated as that where both phases are present in approximately the same amount. The fact already mentioned that no significant changes were observed in spectra taken at room tempera- ture before and after a thermal treatment at a temperature higher than the transition one, indicates that this process is reversible. The hyperfine interaction of the low temperature phase suggests the existence of two inequivalent structural sites for Zr in (NH~)zZrF ~. The temperature dependence of the asymmetry parameters and quad- rupole frequencies of this phase can not be understood in terms of a simple isotropic lattice expansion. Probably, simultaneous effects of expansion and reorientation might explain the different behaviours of both sites. The temperature range at which the high temperature phase is ob- served does not allow us to establish the thermal tendency of its hyperfine parameters. Finally, we would like to mention the fact that the beginning of the phase transition observed near 343 K, being simultaneous with the arrival to full asymmetry of one of the sites in the low-temperature phase, has attracted our attention. This situation has already been observed in other works performed on KzZrF~(2), K~HfF~(5) and (NH,)zHfF~(6) Whether a common physical phenomenon is connected to these processes is open to question. The authors are much indebted to Dr. Graciela Punte and Lic. Silvia Spinelli in obtaining X-ray powder diffraction spectra and for helpful discussions. Partial financial support by CONICET, CICPBA and SUBCyT (Argentina) and Kernforschungszentrum Karlsruhe, GmbH (West Germany) is gratefully acknowledged. References [i] J.A. Martinez, A.M. Rodriguez, M.C. Caracoche, R.C. Mercader and A.R. Lopez Garcia, Hyp. Int. 11(1981)261. [2] J.A. Martinez, A.M. Rodriguez, M.C. Caracoche, R.C. Mercader, A.R. Lopez Garcia and P.C. Rivas, Hyp. Int. 13(1983)307. [3] J.A. Martinez, MoC. Caracoche, A.M. Rodriguez, P.C. Rivas and A.R. Lopez Garcia, Hyp. Int. 14(1983)135. [4] B. Gaudreau, Rev. Chim. Minerale 2(1965)]. [5~ W.V. Prestwich, I.A. Cunningham and T.J. Kennett, Hyp. Int. 12(1982)329. [6] A.M. Rodr~guez, J.A. Martinez, M.C. Caracoche, P.C. Rivas and A.R. Lopez Garcia, to be published. 

Temperature defendence of the hyperfine interaction in (NH4)2ZrF6

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Temperature defendence of the hyperfine interaction in (NH4)2ZrF6

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