Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the initial growth of titanium nitride (TiN) thin films on single-crystal (100) MgO substrates by pulsed laser deposition (PLD). This is the first RHEED study where the growth of TiN films is produced by PLD directly from a TiN target. At the initial stage of the growth (average thickness ∼2.4 nm) the formation of islands is observed. During the continuous growth the islands merge into a smooth surface as indicated by the RHEED, atomic force microscopy and field emission scanning electron microscopy. These observations are in good agreement with the three-dimensional Volmer–Weber growth type, by which three-dimensional crystallites are formed and later cause a continuous surface roughening. This leads to an exponential decrease in the intensity of the specular spot in the RHEED pattern as well.

Growth of thin films of TiN on MgO(100) monitored by high pressure RHEED

Materials with zero linear compressibility (ZLC) can counterintuitively keep its crystal size along a specific direction under hydrostatic high pressure, which makes ZLC materials novel promising functional materials for compression resistance. Herein, for the first time, we propose a modular “dumbbell”-like model for ZLC material design, which is confirmed by three representative materials MCN2 (M = Ca, Fe or Li2) consisting of different structural units. Density functional theory (DFT) calculations reveals that the ZLC effect occurs along the alternatively stacking direction of the negatively compressed NM pyramid and the positively compressed N–C–N pillar. Our finding indicates that a “dumbbell”-like structure endows materials ZLC property; meanwhile, the ZLC can be furtherly tuned in value by submodule substituting. Moreover, “dumbbell” ZLC candidates can be easily discovered, because of the high symmetry and simplicity of the model. The findings provide new insights into the mechanism of ZLC phenomenon and is inspirational for seeking and designing new ZLC materials. 

Tunable Zero Linear Compressibility under a Rational Designed Mechanism of Modular “Dumbbell”: A Density Functional Theory Study

Low dielectric loss and infrared emissivity coating with enhanced mechanical property by grafting modification of EPDM

Theoretical investigations for MFe2 (M = Y, La) Laves alloy and hydrides HxMFe2: Structural stability, lattice thermodynamics, and elastic properties

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 CaTiF6 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 Grüneisen parameters, it is found that the low-frequency phonons, especially acoustic phonons, contribute greatly to the NTE behavior of CaTiF6. The rigid unit modes (RUMs) of low-frequency optical phonons can be identified. The phonon lifetime of CaTiF6 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 CaTiF6.

Anharmonic Interaction in Negative Thermal Expansion Material CaTiF6.

Enhanced thermal stability of Mo film with low infrared emissivity by a TiN barrier layer

Zero thermal expansion (ZTE) and zero linear compressibility (ZLC) are unique and rare properties. Materials combining ZTE and ZLC will have promising prospects. A novel route is proposed in this work to design the coexistence of uniaxial-ZTE and ZLC based on layered hybrid semiconductors [ZnTe(L)0.5] [L = N2H4, ethylenediamine (en), propyldiamine (pda)]. In the framework of [ZnTe(L)0.5], the organic chain contains the attractive and repulsive interactions that arise from the different organic components. It is demonstrated that changing the length of the organic chain can effectively regulate the interaction between different organic components and then achieve the uniaxial-ZTE and ZLC or the desired thermal expansion and compression behaviors. The origin of the coexistence of abnormal axial responses has been traced from thermodynamic formalisms, model Gruneisen parameters and specific vibration modes. It is found that low-energy phonons play an important internal role in realizing the multi-peculiar properties.

Binghui Sun

Papers

Enhanced thermal stability of Mo film with low infrared emissivity by a TiN barrier layer

Adjustable uniaxial zero thermal expansion and zero linear compressibility in unique hybrid semiconductors: the role of the organic chain