Spontaneous strain variations through the low temperature phase transitions of deuterated lawsonite
Spontaneous strain variations through the low temperature phase transitions of deuterated lawsonite
复制标题
氘代硬铝石低温相变引起的自发应变变化
DOI：
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发表时间：
2003
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通讯作者：
K. Knight
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作者：
M. Carpenter;H. Meyer;P. Sondergeld;S. Marion;K. Knight
用高分辨中子粉末衍射法测定了在1.6K-505K温度范围内，在CMCM-↔Pmcn和Pmcn-↔-P21-cn相变过程中，卤水钠铝石的晶格参数变化，这种变化是可逆的。应变分析基于饱和温度下的相变的位移模型来描述T→0K的与温度无关的行为，结果表明，在271±2K的三临界相变和155±1K的二级相变是一致的。与已公布的天然(氢化)样品的膨胀数据相比较，突出了相变最依赖于结构中质子行为的方面。用D替代H会使Pmcn相变的低温相变点增加~27K，应变演化反常从~225 K增加到~250 K。高温相变的相变点在±2 K内保持不变。我们得出结论：在271K相变中，质子有序和位移贡献都是重要的，尽管位移分量提供了初始的对称性破缺机制。更多地依赖于H或D行为的结构变化在这一温度下变得重要约20-50K。Pmcn↔P21cn相变的应变与单序参数驱动的相变是一致的。这些数据被用来确定用于计算相变引起的弹性异常的应变/序参数耦合系数的值。
Abstract High resolution neutron powder diffraction has been used to determine lattice parameter variations of deuterated lawsonite across the Cmcm ↔ Pmcn and Pmcn ↔ P21cn phase transitions, in the temperature interval 1.6-505 K. The variations are reversible through heating and cooling cycles. Strain analysis, based on a displacive model of the transitions with saturation temperatures to describe the temperature-independent behavior as T → 0 K, shows that the data are consistent with a tricritical transition at 271 ± 2 K and a second-order transition at 155 ± 1 K. Comparisons with strains from published dilatation data for a natural (hydrogenated) sample highlight aspects of the transitions that are most dependent on the behavior of protons in the structure. Replacing H by D causes the low temperature transition point to be increased by ~27 K and an anomaly in the strain evolution of the Pmcn transition to increase from ~225 to ~250 K. The transition point of the hightemperature transition remains the same within ± ~2 K. We conclude that proton ordering and displacive contributions are both important in the 271 K transition, though with the displacive component providing the initial symmetry-breaking mechanism. Structural changes that are more dependent on the behavior of H or D become important ~20-50 K below this. Strains for the Pmcn ↔ P21cn transition are consistent with a transition driven by a single order parameter. The data are used to determine values for strain/order parameter coupling coefficients for calculation of elastic anomalies due to the phase transitions.