Although the quasi-harmonic approximation (QHA) method applies to many materials, it is necessary to study the anharmonic interaction for extremely anharmonic materials. In this work, the unusual negative thermal expansion (NTE) property of CaTiF 6 is studied by combing QHA and anharmonic interaction. The improved self-consistent phonon approximation (ISCPA), which treats anharmonic effects in solids nonperturbatively, is employed. The agreement of NTE behavior between the calculation and the experiment can be further promoted from qualitative consistency by QHA to quantitative consistency by the ISCPA. From mode Gruneisen parameters, it is found that the low-frequency phonons, especially acoustic phonons, contribute greatly to the NTE behavior of CaTiF 6 . The rigid unit modes (RUMs) of low-frequency optical phonons can be identified. The phonon lifetime of CaTiF 6 is calculated from three-phonon interactions; thereby, the NTE mechanism can be further explored by phonon lifetimes of phonons with different frequencies on heating. The anomalous lattice thermal conductivity (LTC) is predicted using the Boltzmann transport equation within the relaxation time approximation. The glasslike LTC can occur in crystal CaTiF 6 .

We report ab-initio calculations of water absorption in Y2Mo3O12. The absorption geometry of H2O in Y2Mo3O12 and the binding property between H2O and Y2Mo3O12 have been first identified. Our calculated results show that water is chemisorbed in Y2Mo3O12 with O of the water binding to the Y3+ cation, which is further strengthened by hydrogen bonding between each of the hydrogen atoms of H2O and the bridge O in Y2Mo3O12, shared by polyhedrons YO6 and MoO4. The absorption of water leads to a reduced angle of Y-O-Mo and shortened Y-Mo distance, and consequently volume contraction of the material, almost linearly with the increasing number of water molecules per unit cell, up to eight in total. In addition, our phonon calculation show that the transverse vibration of Y-O-Mo is restricted due to water absorption, which in turn hinders the NTE, as it is mainly originated from this vibrational mode. Our results clarify further the fundamental mechanisms of the large volume shrinkage and the lost NTE of the framework oxide due to water absorption.

Nonplanar penta-graphene and planar phagraphene, which are connected by carbon pentagons and penta-hexa-hepta carbon rings, respectively, are two allotropes of graphene. Graphene as a star material in two-dimensional (2D) materials has been widely studied. However, the studies around penta-graphene and phagraphene are still insufficient. We are interested in both materials' response to temperature, hydrostatic pressure, and stress. In this work, the thermal expansion, linear compressibility, and Poisson’s ratio of penta-graphene and phagraphene have been investigated systematically. It is found that both materials can exhibit abnormal negative thermal expansion (NTE) behavior, while their linear compressibility behavior is normal. The negative Poisson's ratio (NPR) behavior only occurs in penta-graphene, which is consistent with other work. Through an analysis of the lattice vibrations and associated mode Grüneisen parameters, it is found that there are anomalies in the phonon spectra of both penta-graphene and phagraphene. It is noted that acoustic phonons contribute most to their respective anomalies, especially the transverse acoustic (ZA) mode. The librational motion of the lowest-frequency optical phonon of both materials is identified and also associated with their novel properties. In general, the unique topological arrangement of carbon atoms can play a decisive role in determining the performances of penta-graphene and phagraphene.

Although tuning band structure of optoelectronic semiconductor-based materials by means of doping single defect is an important approach for potential photocatalysis application, C-doping or oxygen vacancy (Vo) as a single defect in ZnO still has limitations for photocatalytic activity. Meanwhile, the influence of co-existence of various defects in ZnO still lacks sufficient studies. Therefore, we investigate the photocatalytic properties of ZnO x C0.0625 (x = 0.9375, 0.875, 0.8125), confirming that the co-effect of various defects has a greater enhancement for photocatalytic activity driven by visible-light than the single defect in ZnO. To clarify the underlying mechanism of co-existence of various defects in ZnO, we perform systematically the electronic properties calculations using density functional theory. It is found that the co-effect of C-doping and Vo in ZnO can achieve a more controllable band gap than doping solely in ZnO. Moreover, the impact of the effective masses of ZnO x C0.0625 (x = 0.9375, 0.875, 0.8125) is also taken into account. In comparison with heavy Vo concentrations, the light Vo concentration (x = 0.875) as the optimal component together with C-doping in ZnO, can significantly improve the visible-light absorption and benefit photocatalytic activity.

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