Heat-capacity behaviour of hemimorphite, Zn4Si2O7(OH)2H2O, and its dehydrated analogue Zn4Si2O7(OH)2: a calorimetric and thermodynamic investigation of their phase transitions
Dachs, Edgar; Geiger, Charles A.
European Journal of Mineralogy Volume 21 Number 5 (2009), p. 971 - 983
published: Oct 1, 2009
ArtNo. ESP147052105004, Price: 29.00 €
The heat capacity, Cp, of hemimorphite, Zn4Si2O7(OH)2·H2O, was measured using relaxation calorimetry and DSC methods in the temperature range 5 to 464 K and that of dehydrated hemimorphite, Zn4Si2O7(OH)2, from 5 to 764 K. The experimental Cp data for hemimorphite show a prominent λ-anomaly at 101.8 K that is related to a structural phase transition. An additional weak Cp anomaly occurs around 40 K, suggesting a possible second phase transition. The Cp data of Zn4Si2O7(OH)2 exhibit a λ-anomaly at 86.3 K. At T > 280 K the Cp behaviour of this phase is given by: CZn4Si2O7(OH)2p = 537.7 - 3693.2 T-0.5 - 5.7766 · 106 · T-2 + 7.80821 · 108 · T-3. Two different model approaches were undertaken to describe low-temperature Cp behaviour and to derive phase-transition thermodynamic properties for both phases. In the first approach, heat capacities outside the region of the respective phase transitions (i.e., below 50 K and above 120 K for hemimorphite and above 110 K for Zn4Si2O7(OH)2) were modelled using a combination of Debye, Einstein and Schottky functions. The excess heat capacity for the transition, ΔCp, was then calculated by subtracting interpolated model Cp values from the experimental heat capacities in the temperature region of the λ-anomaly. In the second model approach, the heat capacity of the high-temperature phase was extrapolated into the stability field of the low-temperature phase by use of the Komada-Westrum model. The model Cp values give “base-line” Cp behaviour in the temperature region of the λ-anomaly. A Landau analysis shows that the transitions in both phases are principally first order in character, but are close to a tricritical point with Tc = 101.8 K for hemimorphite and Tc = 86.3 K for Zn4Si2O7(OH)2. The excess heat capacity, ΔCp, was fitted to a tricritical Landau expression ΔCp = aT/(4√Tc(Tc - T)) and the determined thermodynamic phase-transition properties are ΔHtr = 494 ± 13 J/mol and ΔStr = 7.3 ± 0.3 J/mol·K for hemimorphite and ΔHtr = 360 ± 11 J/mol and Str = 6.3 ± 0.2 J/mol·K for Zn4Si2O7(OH)2 (a = 14.6 1.4 J/mol·K for hemimorphite and a = 12.5 ± 0.8 J/mol·K for Zn4Si2O7(OH)2). A possible crystal-chemical explanation for the transition in both phases is that dynamic proton disorder, associated with the OH groups of the framework in the high-temperature phase, is quenched below the transition temperature. Around 40 K ΔCp behaviour for hemimorphite is not described well by a Landau model, thus indicating a second phase transition. Its excess Cp, in addition to the Landau ΔCp, gives ΔHtr = 86 ± 3 J/mol and ΔStr = 1.8 ± 0.1 J/mol·K. This transition at ∼40 K could be related to changes in the weak H-bonding arrangement in the micropores of hemimorphite involving the H2O molecules. This proposal is strengthened by the fact that a similar transition is not observed in Zn4Si2O7(OH)2. Standard entropies, obtained using the first model approach are So = 369.5 ± 3.0 J/mol·K for hemimorphite and So = 315.8 ± 2.5 J/mol·K for Zn4Si2O7(OH)2. These values agree within error with those derived from the second model approach.