Dachs Edgar, Benisek Artur, Harlov Daniel, Wilke Max
Fachbereich Chemie und Physik der Materialien, Universität Salzburg, Jakob-Haringerstrasse 2a, 5020 Salzburg, Austria.
Deutsches GeoForschungsZentrum-GFZ, Telegrafenberg, 14473 Potsdam, Germany.
Phys Chem Miner. 2021;48(12):44. doi: 10.1007/s00269-021-01167-1. Epub 2021 Oct 28.
The heat capacity, , of synthetic hydroxyapatite [Ca(PO)OH-OH-Ap], as well as of ten compositions along the OH-Ap-chlorapatite (Cl-Ap) join and 12 compositions along the OH-Ap-fluorapatite (F-Ap) join have been measured using relaxation calorimetry (heat capacity option of the Physical Properties Measurement System-PPMS) and differential scanning calorimetry (DSC) in the temperature range of 5-764 K. Apatites along the Cl-OH and F-OH joins were synthesized at 1100 °C and 300 MPa in an internally heated gas pressure vessel via an exchange process between synthetic fluorapatite or chlorapatite crystals (200-500 μm size) and a series of Ca(OH)-HO solutions with specific compositions and amounts relative to the starting apatite. The standard third-law entropy of OH-Ap, derived from the low-temperature heat capacity measurements, is ° = 386.3 ± 2.5 J mol K, which is ~ 1% lower than that resulting from low-temperature adiabatic calorimetry data on OH-Ap from the 1950's. The heat capacity of OH-Ap above 298.15 K shows a hump-shaped anomaly centred around 442 K. Based on published structural and calorimetric work, this feature is interpreted to result from a monoclinic to hexagonal phase transition. Super ambient up to this transition can be represented by the polynomial: . The DSC data above this transition were combined with heat capacities computed using density functional theory and can be given by the polynomial: . Positive excess heat capacities of mixing, ∆ , in the order of 1-2 J mol K, occur in both solid solutions at around 70 K. They are significant at these conditions exceeding the 2σ-uncertainty of the data. This positive ∆ is compensated by a negative ∆ of the same order at around 250 K in both binaries. At higher temperatures (up to 1200 K), ∆ is zero within error for all solid solution members. As a consequence, the calorimetric entropies, S, show no deviation from ideal mixing behaviour within a 2σ-uncertainty for both joins. Excess entropies of mixing, ∆S, are thus zero for the OH-Ap-F-Ap, as well as for the OH-Ap-Cl-Ap join. The - behaviour of the OH-Ap endmember is discussed in relation to that of the F- and Cl-endmembers.
The online version contains supplementary material available at 10.1007/s00269-021-01167-1.
使用弛豫量热法（物理性质测量系统 - PPMS的量热容量选项）和差示扫描量热法（DSC），在5 - 764 K的温度范围内，测量了合成羟基磷灰石[Ca(PO)OH - OH - Ap]以及沿OH - Ap - 氯磷灰石（Cl - Ap）连接的十种成分和沿OH - Ap - 氟磷灰石（F - Ap）连接的十二种成分的热容量。沿Cl - OH和F - OH连接的磷灰石是在内部加热的气体压力容器中，于1100°C和300 MPa的条件下，通过合成氟磷灰石或氯磷灰石晶体（尺寸为200 - 500μm）与一系列具有特定组成和相对于起始磷灰石的量的Ca(OH) - H₂O溶液之间的交换过程合成的。从低温热容量测量得出的OH - Ap的标准第三定律熵为° = 386.3 ± 2.5 J mol⁻¹ K⁻¹，这比20世纪50年代关于OH - Ap的低温绝热量热法数据得出的值低约1%。OH - Ap在298.15 K以上的热容量显示出以442 K为中心的驼峰状异常。基于已发表的结构和量热工作，此特征被解释为单斜相到六方相的转变。直至该转变的超环境 可以用多项式表示： 。该转变以上的DSC数据与使用密度泛函理论计算的热容量相结合，可以用 多项式表示： 。在两种固溶体中，在约70 K时出现了数量级为1 - 2 J mol⁻¹ K⁻¹的正混合超额热容量∆ 。在这些条件下，它们很显著，超过了数据的2σ不确定性。在两种二元体系中，在约250 K时，这种正的∆ 被相同数量级的负∆ 所补偿。在较高温度（高达1200 K）下，对于所有固溶体成员来说，∆ 在误差范围内为零。因此，对于两种连接，量热熵S在2σ不确定性内均未显示出与理想混合行为的偏差。因此，对于OH - Ap - F - Ap以及OH - Ap - Cl - Ap连接，混合超额熵∆S均为零。讨论了OH - Ap端元的 - 行为与F - 和Cl - 端元的行为的关系。
在线版本包含可在10.1007/s00269 - 021 - 01167 - 1获取的补充材料。
