DSC Investigations of the Phase Transitions of [M(NH 3 ) 6 ](C10 4 ) 2 and [M(NH 3 ) 6 ](BF 4 ) 2, where M = Co and Cd

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1 DSC Investigations of the Phase Transitions of [M(NH 3 ) 6 ](C10 4 ) 2 and [M(NH 3 ) 6 ](BF 4 ) 2, where M = Co and Cd A. Migdał-Mikuli, E. Mikuli, S. Wróbel 3, and Ł. Hetmańczyk Department of Chemical Physics, Faculty of Chemistry of the Jagiellonian University, ulica Ingardena 3, Krakow. Poland a Department of Solid State Physics, M. Smoluchowski Institute of Physics, Jagiellonian University, ulica Reymonta 4, Krakow, Poland Reprint requests to Dr. A. M.-M.; mikuli@trurl.ch.uj.edu.pl Z. Naturforsch. 54 a, (1999); received August 13, 1999 Solid polymorphism of four compounds of the type [M(NH 3 ) 6 ](XY 4 ) 2, where M = Co 2+ or Cd 2+, and XY 4 = C10 4 ~ or BF 4 ~ has been studied at K by DSC. One or two phase transitions of the investigated compounds have been found. For the compounds with M = Co the phase transitions have not yet been described in the literature. For the compounds with M = Cd the phase transition-temperature is in good agreement with the results obtained by NMR and EPR. Generally, for [M(NH 3 ) 6 ](BF 4 ) 2 compounds (M = Mg, Fe, Co, or Ni) the phase transition temperature r c, is lower than that for the corresponding [M(NH 3 ) 6 ](C10 4 ) 2, but for compounds with M = Cd it is higher. However, the enthalpy and entropy changes at the T CI phase transitions of [M(NH 3 ) 6 ](BF 4 ) 2 are always lower than those for [M(NH 3 ) 6 ](C10 4 ) 2. Moreover, for the compounds of this type a correlation between the transition temperature T C] and the crystal lattice parameter a has been found. Key words: Phase Transitions; DSC Method; Chlorate(VII) and Tetrafluoroborate of Hexaaminacobalt(II) and Hexaaminacadmium(II). 1. Introduction Adiabatic calorimetry studies of [M(NH 3 ) 6 ]- (XY 4 ) 2, where M = Ni 2+ or Mg 2+, and XY 4 = C10 4 " or BF 4 ~, revealed two phase transitions [1-3]. The transition from high temperature to the intermediate phase shows at T cl a large specific heat anomaly, whereas the transition from the intermediate to the low temperature phase shows at T c2 a much smaller specific heat anomaly. Extensive studies by various methods [3-22] established that these phase transitions are associated both with a change of the crystal structure and with an abrupt change in the rate of reorientational motion of the tetrahedral anions. Fast reorientational motions of the NH 3 groups take place in all three phases of these compounds and are only slightly distorted by the phase transitions. Their characteristic correlation time amounts to several picoseconds even at liquid nitrogen temperature [21]. The phase transition at T cl was also observed in [M(NH 3 ) 6 ](XY 4 ) 2 compounds with M = Fe or Cd by Mössbauer [23], NMR [24, 25] and EPR [26] methods. For the com- pounds with M = Mn or Co neither phase transitions nor reorientational motions have yet been studied. So far a few theoretical attempts at a description of the phase transitions in the discussed compounds have been done [27-31]. Unfortunately, none of them has predicted correctly the phase transition temperatures and the observed monoclinic distortion of the crystal. However, taking into account the symmetry of the anion, it is possible to correlate T C] of the highest temperature phase transition with the lattice parameters of these crystals [28]. The [M(NH 3 ) 6 ](XY 4 ) 2 crystalline compounds, have at room temperature a face centered cubic structure (space group Fm3m - O^, no. 225) with four molecules in the unit cell [32-34]. The high symmetry of the crystal lattice is possible because the NH 3 groups perform fast reorientational motions around the threefold axis. Both the intermediate and low temperature phases of these compounds are monoclinic (space group P2,/c - C 2h, no. 14) with four molecules in the unit cell [3, 6, 14, 16, 17] / 99 / $ Verlag der Zeitschr ift für Naturforschung, Tübingen

2 A. Migdal-Mikuli et al. DSC Investigations of Phase Transitions, I T h e a i m of this study is to detect a n o m a l i e s of the D S C c u r v e s a n d h e n c e to d e t e r m i n e the therm o d y n a m i c p a r a m e t e r s of the p h a s e transitions in [M(NH3)6](C104)2 and [M(NH3)6](BF4)2, where M = C o or C d. Additionally, w e will present the correlation b e t w e e n Tcl a n d the lattice p a r a m e t e r a for [ M ( N H 3 ) 6 ] ( X Y 4 ) 2 c o m p o u n d s with M = M g, Fe, Co, Ni or C d. 2. Experimental T h e e x a m i n e d c o m p o u n d s w e r e obtained f r o m c o r r e s p o n d i n g h e x a a q u a m e t a l ( I I ) chlorates(vii) and tetrafluoroborates w h i c h w e r e earlier synthesized by reaction of the c o r r e s p o n d i n g c a r b o n a t e s with diluted H C and H B F 4, respectively, and recrystallized several t i m e s f r o m w a t e r distilled f o u r times in a q u a r t z vessel. T h e a d e q u a t e hexaaquametal(ii) c o m p l e x p l a c e d in a q u a r t z vessel w a s put in a glass tube t h r o u g h w h i c h dry g a s e o u s a m m o n i a was b l o w n, a n d the t u b e w a s p l a c e d inside an o v e n. First the tube w a s h e a t e d f o r several h o u r s at about 390 K until all the w a t e r f r o m t h e h e x a a q u a c o m p l e x had been lost. T h e n the c o r r e s p o n d i n g h e x a a m m i n e c o m p l e x that f o r m e d w a s c o o l e d to r o o m t e m p e r a t u r e and kept at this t e m p e r a t u r e f o r several d a y s. All h e x a a m m i n e c o m p o u n d s e x h i b i t p o o r stability at r o o m t e m p e r a t u r e if they are not k e p t in a c o n t a i n e r filled with dry a m m o n i a. B e f o r e t h e m e a s u r e m e n t s, the c o m p o s i t i o n of the c o m p o u n d s w a s d e t e r m i n e d on the basis of their m e t a l a n d a m m o n i a content, by titration by m e a n s of n a t r i u m w e r s e n a t e a n d h y d r o c h l o r i c acid, respectively. T h e a v e r a g e c o n t e n t s of m e t a l and N H 3 w e r e f o u n d to b e e q u a l to the theoretical values within the error limit of ca. 10%. T h e D S C m e a s u r e m e n t s w e r e p e r f o r m e d with a P e r k i n - E l m e r P Y R I S 1 D S C a p p a r a t u s at the M. S m o l u c h o w s k i Institute of P h y s i c s of the Jagiellonian University. T h e i n s t r u m e n t w a s calibrated by m e a n s of the m e l t i n g p o i n t of i n d i u m - f o r the high temp e r a t u r e region, a n d the m e l t i n g point of H for the l o w t e m p e r a t u r e region. H i g h purity dry gases w e r e used as p u r g i n g gas ( h e l i u m, % ) and as air shield g a s (nitrogen, % ). T h e nitrogen gas used for t r a n s f e r r i n g liquid nitrogen to the cold finger d e w a r w a s also of high purity. T w o characteristic t e m p e r a t u r e s of t h e D S C p e a k s, obtained on heating, w e r e c o m p u t e d : t h e t e m p e r a t u r e of the peak m a x i m u m (T p e a k ) a n d the t e m p e r a t u r e calculated f r o m the slope of the l e f t - h a n d side of the p e a k (T o n s e t ). T h e s e 591 two temperatures differed by 2 to 6 K. In the c a s e of second order transitions also the T e n d t e m p e r a t u r e was c o m p u t e d f r o m the right-hand side p e a k slope. For first order transitions the T o n s e t and T e n d t e m p e r a tures obtained m a y differ by 1 to 3 K, w h e r e a s f o r the second order transitions the d i f f e r e n c e m a y b e as high as 10 K, or even greater. F o r sharp p e a k s the value of T o n s e t and for d i f f u s e p e a k s the value of T p e a k as the phase transition t e m p e r a t u r e w a s taken into a c c o u n t, respectively. T h e enthalpy c h a n g e s ( A H ) c o n n e c t e d with the observed p h a s e transitions w e r e c a l c u l a t e d by numerical integration of the D S C c u r v e s u n d e r the peaks of the a n o m a l i e s. B e f o r e the c a l c u l a t i o n s a linear b a c k g r o u n d w a s subtracted. T h i s w a s d o n e in a m o r e or less arbitrary t h o u g h identical w a y f o r all the samples. Nevertheless, they are g o o d e n o u g h to allow the c o m p a r i s o n of the c o m p o u n d s investigated. T h e entropy c h a n g e s (AS) w e r e calculated u s i n g the f o r m u l a AS x = A H / T C x. F o r the sharp p e a k s of the D S C curves they w e r e c o m p u t e d with h i g h a c c u r a c y ( ± 4 % ), w h e r e a s for the d i f f u s e p e a k s they c o u l d b e considered as estimates only. T h e D S C m e a s u r e m e n t s were p e r f o r m e d on heating a n d c o o l i n g the s a m p l e s with a constant rate of 10 K - m i n - 1. T h e m a s s e s of t h e samples a m o u n t e d to 8 to 28 m g. 3. Results and Discussion Figure 1 s h o w s as an e x a m p l e the t e m p e r a t u r e dependencies of the heat flow ( D S C c u r v e s ) o b t a i n e d during the heating and c o o l i n g of the [ C d ( N H 3 ) 6 ] ( C ) 2 s a m p l e at the rate of 10 K - m i n - 1. A sharp peak on both D S C curves is seen. It c o r r e s p o n d s to a solid-solid p h a s e transition. T h e d i f f e r e n c e b e t w e e n the T o n s e t t e m p e r a t u r e s o b t a i n e d on h e a t i n g a n d c o o l - ing is ca. 5 K, so a small hysteresis of the phase transition t e m p e r a t u r e Tcl w a s detected. It indicates that this p h a s e transition is of first o r d e r. S i m i l a r D S C curves w e r e registered f o r [ C d ( N H 3 ) 6 ] ( B F 4 ) 2, [ C o ( N H 3 ) 6 ] ( C ) 2 and [ C o ( N H 3 ) 6 ] ( B F 4 ) 2 s a m p l e s. T h e c o m p a r i s o n of the D S C curves (registered on heating the s a m p l e s at a rate of 10 K - m i n - 1 ) f o r [ M ( N H 3 ) 6 ] ( B F 4 ) 2 and [ M ( N H 3 ) 6 ] ( C ) 2 is s h o w n in Figs. 2 ( M = Cd) and 3 ( M = Co), respectively. Evidently the p h a s e transition of [ C d ( N H 3 ) 6 ] ( B F 4 ) 2 takes place at a higher t e m p e r a t u r e than that of [ C d ( N H 3 ) 6 ] ( C ) 2. For the c o r r e s p o n d i n g cobalt c o m p o u n d s it is the opposite. W e believe that f o r t h e s e there exists also a very small and d i f f u s e d a n o m a l y on the D S C c u r v e in the low t e m p e r a t u r e region. F o r

3 592 A. Migdal-Mikuli et al. DSC Investigations of Phase Transitions, I [Cd(NH 3 ) 6 ](CI0 4 ) [Co(NH 3 ) 6 ](BF 4 ) 2 2 [Co(NH 3 ) 6 ](CI0 4 ) i 4 H 1 3H ro a I -4-6 to, j <D I I Fig. 1. DSC curves for [Cd(NH 3 ) 6 ](C10 4 ) 2 registered on cooling and heating at a rate of 10 K-min. Fig. 3. Comparison of the DSC curves for [Co(NH 3 ) 6 ]- (C10 4 ) 2 and [Co(NH 3 ) 6 ](BF 4 ) 2 registered on heating at a rate of 10 K-min [Cd(NH 3 ) 6 ](CI0 4 ) 2 2 [Cd(NH ) ](BF ) [M(NH3),](CI04) H I [M(NH,),](BF«)j TO c a) 6 X Fig. 2. Comparison of the DSC curves for [Cd(NH 3 ) 6 ]- (C10 4 ) 2 and [Cd(NH 3 ) 6 ](BF 4 ) 2 registered on heating at a rate of 10 K-min Lattice constant [A] Fig. 4. Phase transition temperature TCI vs. lattice constant a for [Me(NH 3 ) 6 ](XY 4 ) 2 compounds. [CO(NH 3 ) 6 ](C10 4 ) 2 this very small anomaly at ca. 140 K is quite evident on the DSC curve of Fig. 3. This observation will be a subject of our further

A. Migdal-Mikuli et al. DSC Investigations of Phase Transitions, I 593 Table 1. Thermodynamic parameters of the phase transitions of [M(NH 3 ) 6 ](C10 4 ) 2 and [M(NH 3 ) 6 ](BF 4 ) 2.

4 A. Migdal-Mikuli et al. DSC Investigations of Phase Transitions, I 593 Table 1. Thermodynamic parameters of the phase transitions of [M(NH 3 ) 6 ](C10 4 ) 2 and [M(NH 3 ) 6 ](BF 4 ) 2. M rc[k] Mg Cd Cd 125 Fe Co Ni -[M(NH3)6](C104)2 AH [kj-mol - 1 ] AS [J m o l _ 1 K _ 1 ] Ref. r c [K] [3] [3] [23] [2] [2] [25] investigations. T h e t e m p e r a t u r e and e n t h a l p y - and e n t r o p y - c h a n g e s f o r the p h a s e transitions detected f o r all the investigated c o m p o u n d s are presented in Table 1 together with the literature data on the analo g o u s c o m p o u n d s with M = M g, F e a n d Ni. Table 1 s h o w s that AH and AS f o r the [ M ( N H 3 ) 6 ] ( B F 4 ) 2 c o m p o u n d s are distinctly lower than t h o s e for the [ M ( N H 3 ) 6 ] ( C ) 2 o n e s. F o r c o m p o u n d s with M = Cd there is a possibility of a c o m p a r i s o n of the transition t e m p e r a t u r e s o b t a i n e d in this w o r k with those obtained b y N M R [24, 25] and E P R [26]. T h e a g r e e m e n t of the results is m u c h better f o r [ C d ( N H 3 ) 6 ] ( B F 4 ) 2 than for [ C d ( N H 3 ) 6 ] ( C ) 2. Figure 4 s h o w s a correlation b e t w e e n the high p h a s e transition t e m p e r a t u r e (Tcl) a n d the crystal lattice parameter (a) f o r all the c o m p o u n d s presented in Table 1. A similar correlation f o r [ M ( N H 3 ) 6 ] X 2 c o m p o u n d s with different a n i o n s and different metal cations w a s noticed by S t a n k o w s k i [31]. O u r w o r k improves this correlation f o r [ C d ( N H 3 ) 6 ] ( B F 4 ) 2 and [ C d ( N H 3 ) 6 ] ( C ) 2 and adds t w o new points, n a m e l y for [ C o ( N H 3 ) 6 ] ( B F 4 ) 2 and [ C o ( N H 3 ) 6 ] ( C ) 2. It c a n b e seen in Fig. 4 that for the [ M ( N H 3 ) 6 ] ( B F 4 ) 2 c o m p o u n d s Tcl slightly increases w i t h increasing lattice constant a, w h i l e f o r [ M ( N H 3 ) 6 ] ( C ) 2 it is quite the opposite: TCI strongly d e c r e a s e s with increasing a. [ M ( H 2 0 ) 6 ] ( C ) 2 b e h a v e s in an o t h e r w a y than m o s t [ M ( N H 3 ) 6 ] X 2 c o m p o u n d s. W e are yet u n able to explain this d i f f e r e n c e. W e did not o b s e r v e such a d i f f e r e n c e in the c a s e of the similar f a m i l i e s [1] T. Grzybek, J. A. Janik, J. Mayer, G. Pytasz, M. Rachwalska, and T. Waluga, Phys. Stat. Sol. (a) 16, K165 (1973). [2] M. Rachwalska, J. M. Janik, J. A. Janik, G. Pytasz, and T. Waluga, Phys. Stat. Sol. (a) 30, K81 (1975). [M(NH 3 ) 6 ](BF 4 ) 2 AH [kj-mol - 1 ] AS [ J m o l - 1 K - 1 ] Ref. [23] [1] [1] 147 [M(H20)6](C104)2 either. [24, 26] and [M(H20)6](BF4)2 [35], 4. C o n c l u s i o n s T h e results of and their c o m p a r i s o n w i t h the literature data lead to the f o l l o w i n g c o n c l u s i o n s : 1. T h e D S C curves of all the investigated c o m p o u n d s are very similar to the t e m p e r a t u r e d e p e n d e n c e of the heat capacity of [ N i ( N H 3 ) 6 ] ( C ) 2, [ M g ( N H 3 ) 6 ] ( C ) 2 and [ N i ( N H 3 ) 6 ] ( B F 4 ) T h e p h a s e transition t e m p e r a t u r e s f o r the [ M ( N H 3 ) 6 ] ( B F 4 ) 2 c o m p o u n d s are generally l o w e r than those f o r the c o m p o u n d s with C ~ a n i o n s. However, for c o m p o u n d s with M = C d it is q u i t e the opposite. 3. T h e enthalpy and entropy c h a n g e s d e t e c t e d f o r the [ M ( N H 3 ) 6 ] ( B F 4 ) 2 c o m p o u n d s are a l w a y s significantly smaller than those for the c o m p o u n d s w i t h C ~ anions. 4. For both [ M ( N H 3 ) 6 ] ( C ) 2 and [ M ( N H 3 ) 6 ] ( B F 4 ) 2 there exists a correlation b e t w e e n Tcl a n d the crystal lattice p a r a m e t e r a. However, w h e r e a s T C 1 of [ M ( N H 3 ) 6 ] ( C ) 2 decreases with increasing a, that of [ M ( N H 3 ) 6 ] ( B F 4 ) 2 increases with increasing a. Acknowledgements O n e of us (S. W.) would like to e x p r e s s his gratitude to the Polish F o u n d a t i o n for S c i e n c e f o r p u r c h a s i n g the P Y R I S 1 D S C calorimeter. [3] A. Migdal-Mikuli, E. Mikuli, M. Rachwalska, and S. Hodorowicz, Phys. Stat. Sol. (a) 47, 57 (1978). [4] J. Stankowski, J. M. Janik, A. Dezor, and B. Sczaniecki, Phys. Stat. Sol. (a) 16, K167 (1973).

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](BF 4. Studied by Differential Scanning Calorimetry

](BF 4. Studied by Differential Scanning Calorimetry Phase Polymorphism of [Mn(DMSO) 6 ](BF 4 ) 2 Studied by Differential Scanning Calorimetry Anna Migdał-Mikuli and Łukasz Skoczylas Department of Chemical Physics, Faculty of Chemistry, Jagiellonian University,