Phase Transitions in Crystalline [Me(H₂0)₆ ](BF₄ )₂. Part I (Me = Mn, Fe, Co, Ni and Zn)

Abstract

Solid polymorphism in five compounds of the type [Me(H20)6](BF4)2 , where Me = Mn, Fe, Co, Ni and Zn, have been studied in the temperature range 120 -330 K by DSC. For each substance at least three solid modifications have been found. In addition, for compounds with Me = Mn, Fe and Zn some new low temperature phase transitions have been observed. The transition temperatures for the [Me(H2O)6](BF4)2 compounds are distinctly lower than those for the [Me(H2O)6](ClO4)2 compounds. The entropy-and enthalpy-changes at the phase transitions of the hexaaquametal(II) tetrafluoroborates are significantly lower than those for the corresponding chlorates(VII). Linear correlations between the highest transition temperature and the crystal density are found for both series of these isomorphous compounds.

Phase Transitions and Water Dynamics of [Co(H2O)6](ClO4)2 and [Mn(H2O)6](BF4)2 Studied by Neutron Scattering Methods

“…For [Co(H20 ) 6](C104)2 we detected by DSC [3] phase transitions at the temperatures: T C1 = 348 K, TC2 = 241 K, and T C3 = 156 K. However, the tem peratures o f the phase transitions were also ascribed to 263 K and 215 K by adiabatic calorimetry [5], to 354 K, 243 K, and 155 K by EPR [6,7], to 156 K by the Mössbauer effect [8], to 160 K by NMR [9], to 166 K by magnetic susceptibility [10,11], to 333 K, 240 K and 166 K by IR and RS [12], and to 344 K, 295 K, 243 K, 182 K, 155 K, 136 K and 121 K by X-ray and dielectric permittivity [13]. For [Mn(H20 ) 6](BF4)2 we detected by DSC [4] the phase transitions at the following temperatures: T r , = 250 K, TC7 = 223 K and T C3 = 162 K.…”

“…Phase transitions in these compounds have been studied by many methods, however there is a consid erable disagreement concerning the transition tem peratures and even the number of phases, especially in the case o f compounds with M = Mn, Fe, and Co [3,4]. For [Co(H20 ) 6](C104)2 we detected by DSC [3] phase transitions at the temperatures: T C1 = 348 K, TC2 = 241 K, and T C3 = 156 K. However, the tem peratures o f the phase transitions were also ascribed to 263 K and 215 K by adiabatic calorimetry [5], to 354 K, 243 K, and 155 K by EPR [6,7], to 156 K by the Mössbauer effect [8], to 160 K by NMR [9], to 166 K by magnetic susceptibility [10,11], to 333 K, 240 K and 166 K by IR and RS [12], and to 344 K, 295 K, 243 K, 182 K, 155 K, 136 K and 121 K by X-ray and dielectric permittivity [13].…”

Thermal properties of polycrystalline [Mn(NH3)6](ClO4)2

“…The first run (sample A, mass = 8.04 mg) was made on a Perkin-Elmer DIAMOND 8000 DSC apparatus, whereas the second run (sample B, mass = 2.83 mg) was made on a Mettler-Toledo 822 e calorimeter, both runs were performed in range of 123-300 K. The DSC measurements were made both during heating and cooling of the sample at constant rates of 10 and 20 K min -1 for sample A and 5 K min -1 for sample B. Other experimental details were the same as those published in our previous paper [12].…”

The EPR Zero-Field Splitting D and its Pressure and Temperature Dependence for Trigonal Mn2+ Centers in [Zn(H2O)6](BF4)2:Mn2+ Crystal

“…n ions have attracted great interest because: (i) there are several phase transitions induced by weak orientation interaction forces in these crystals [1,2]; (ii) these crystals have unusual properties caused by bivalent impurity ions arranged in the form of chains weakly linked to each other [3]; (iii) they are very compressible and can serve as model objects in studies at high pressure [4,5]. So, many EPR spectra for bivalent 3d n ions (e. g., Mn 2+ , Ni 2+ and Co 2+ ) in these crystals have been measured [4 -8].…”