Crystal-chemical and energetic systematics of wadeite-type phases A2BSi3O9 (A = K, Cs; B = Si, Ti, Zr) |
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Authors: | Hongwu Xu Alexandra Navrotsky M Lou Balmer Yali Su |
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Institution: | (1) Thermochemistry Facility and NEAT ORU, University of California at Davis, Davis, CA 95616, USA;(2) Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-93, Battelle Blvd., Richland, WA 99352, USA;(3) Present address: Los Alamos Neutron Science Center, LANSCE-12, MS-H805, Los Alamos National Laboratory, Los Alamos, NM 87545, USA;(4) Present address: Caterpillar Inc., Technical Center, E/854, 1875, IL 61656 Peoria, USA;(5) Present address: Kronos Science Laboratories, 2222 E. Highland Ave., Phoenix, AZ 85016, USA |
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Abstract: | Wadeite K2ZrSi3O9 and its analogues K2TiSi3O9 and Cs2ZrSi3O9, synthesized by high-temperature solid-state sintering, have been investigated using powder X-ray diffraction coupled with
Rietveld analysis and high-temperature oxide melt solution calorimetry. The crystal chemistry and energetics of these phases,
together with K2SiVISi3
IVO9, a high-pressure wadeite analogue containing both tetrahedral and octahedral Si, are discussed in term of ionic substitutions.
As the size of the octahedral framework cation increases, Si4+ → Ti4+ → Zr4+, the cell parameter c increases at a much higher rate than a. In contrast, increasing the interstitial alkali cation size (K+ → Cs+) results in a higher rate of increase in a compared with c. This behavior can be attributed to framework distortion around the interstitial cation. The enthalpies of formation from
the constituent oxides (ΔHf,ox0) and from the elements (ΔHf,el0) have been determined from drop-solution calorimetry into 2PbO·B2O3 solvent at 975 K. The obtained values (in kJ/mol) are as follows: ΔHf,ox0 (K2TiSi3O9) = −355.8 ± 3.0, ΔHf,el0 (K2TiSi3O9) = −4395.1 ± 4.8, ΔHf,ox0 (K2ZrSi3O9) = −374.3 ± 3.3, ΔHf,el0 (K2ZrSi3O9) = −4569.9 ± 5.0, ΔHf,ox0 (Cs2ZrSi3O9) = −396.6 ± 4.4, and ΔHf,el0 (Cs2ZrSi3O9) = −4575.0 ± 5.5. The enthalpies of formation for K2SiVISi3
IVO9 were calculated from its drop-solution enthalpy of an earlier study (Akaogi et al. 2004), and the obtained ΔHf,ox0 (K2SiSi3O9) = −319.7 ± 3.4 and ΔHf,el0 (K2SiSi3O9) = −4288.7 ± 5.1 kJ/mol. With increasing the size of the octahedral framework cation or of the interstitial alkali cation,
the formation enthalpies become more exothermic. This trend is consistent with the general behavior of increasing energetic
stability with decreasing ionic potential (z/r) seen in many oxide and silicate systems. Further, increasing the size of the octahedral framework cation appears to induce
more rapid increase in stability than increasing the interstitial alkali cation size, suggesting that framework cations play
a more dominant role in wadeite stability. |
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Keywords: | Wadeite Crystal chemistry X-ray diffraction Thermodynamics Enthalpy of formation High-temperature calorimetry |
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