Substitution effects on the temperature vs. magnetic-field phase diagrams of the quasi-1D effective Ising spin-1/2 chain system BaCo2_2V2_2O8_8

BaCo$_2$V$_2$O$_8$ is a one-dimensional antiferromagnetic spin-1/2 chain system with pronounced Ising anisotropy of the magnetic exchange. Due to finite interchain interactions long-range antiferromagnetic order develops below $T_{\rm N} \simeq 5.5$ K, which is accompanied by a structural distortion in order to lift magnetic frustration effects. The corresponding temperature $vs.$ magnetic-field phase diagram is highly anisotropic with respect to the magnetic-field direction and various details are still under vivid discussion. Here, we report the influence of several substitutions on the magnetic properties and the phase diagrams of BaCo$_2$V$_2$O$_8$. We investigate the substitution series Ba$_{\text{1-x}}$Sr$_{\text{x}}$Co$_{\text{2}}$V$_{\text{2}}$O$_{\text{8}}$ over the full range $0\le x \le 1$ as well as the influence of a partial substitution of the magnetic Co$^{2+}$ by small amounts of other magnetic transition metals or by non-magnetic magnesium. In all cases, the phase diagrams were obtained on single crystals from magnetization data and/or high-resolution studies of the thermal expansion and magnetostriction.Comment: 10 pages, 10 figure

A review of mechanoluminescence in inorganic solids : compounds, mechanisms, models and applications

Mechanoluminescence (ML) is the non-thermal emission of light as a response to mechanical stimuli on a solid material. While this phenomenon has been observed for a long time when breaking certain materials, it is now being extensively explored, especially since the discovery of non-destructive ML upon elastic deformation. A great number of materials have already been identified as mechanoluminescent, but novel ones with colour tunability and improved sensitivity are still urgently needed. The physical origin of the phenomenon, which mainly involves the release of trapped carriers at defects with the help of stress, still remains unclear. This in turn hinders a deeper research, either theoretically or application oriented. In this review paper, we have tabulated the known ML compounds according to their structure prototypes based on the connectivity of anion polyhedra, highlighting structural features, such as framework distortion, layered structure, elastic anisotropy and microstructures, which are very relevant to the ML process. We then review the various proposed mechanisms and corresponding mathematical models. We comment on their contribution to a clearer understanding of the ML phenomenon and on the derived guidelines for improving properties of ML phosphors. Proven and potential applications of ML in various fields, such as stress field sensing, light sources, and sensing electric (magnetic) fields, are summarized. Finally, we point out the challenges and future directions in this active and emerging field of luminescence research

First-principles modeling of the thermoelectric properties of SrTiO3_3/SrRuO3_3 superlattices

Using a combination of first-principles simulations, based on the density functional theory and Boltzmann's semiclassical theory, we have calculated the transport and thermoelectric properties of the half-metallic two dimensional electron gas confined in single SrRuO$_3$ layers of SrTiO$_3$/SrRuO$_3$ periodic superlattices. Close to the Fermi energy we find that the semiconducting majority spin channel displays a very large in-plane component of the Seebeck tensor at room temperature, $S$ = 1500 $\mu$V/K, and the minority spin channel shows good in-plane conductivity $\sigma$ = 2.5 (m$\Omega$cm)$^{-1}$. However, contrary to the expectation of Hicks and Dresselhaus model about enhanced global thermoelectric properties due to the confinement of the metallic electrons, we find that the total power factor and thermoelectric figure of merit for reduced doping is too small for practical applications. The reason for this failure can be traced back on the electronic structure of the interfacial gas, which departs from the free electron behaviour on which the model was based. The evolution of the electronic structure, electrical conductivity, Seebeck coefficient, and power factor as a function of the chemical potential is explained by a simplified tight-binding model. We find that the electron gas in our system is composed by a pair of one dimensional electron gases orthogonal to each other. This reflects the fact the physical dimensionality of the electronic system can be even smaller than that of the spacial confinement of the carriers.Comment: 9 pages, 7 figure

Solid state NMR studies of inorganic pigment materials and catalysts

The research conducted can be split into three major regions; pigment material, a simulation program called QuadFit and mesoporous oxides. There has also been some extra work conducted on a catalyst for partial hydrogenation of vegetable oils. Various techniques have been used on the different systems including XPS, STEM and 77Se, 17O, 93Nb, 15N, 119Sn, 27Al, 115In and 63,65Cu static and MAS NMR. The pigment materials consist of a series of materials which are grouped under the F-Colours project. The pigments consist of sulphur doped tin niobates, copper indium sulphur selenide doped zinc selenides, sulphur doped tin tungstates and colloidal gold and silver enamels. The sulphur doped tin niobate study shows a conversion from foordite to pyrochlore and also where the sulphur sits in the structure. The copper indium sulphur selenide doped zinc selenide study shows the indium and copper moving into the zinc selenide as copper indium pairs. However, how the pairs sit in the structure remains undetermined. The sulphur doped tin tungstate study shows that the sulphur acts as a promoter for the beta phase rather than the desired alpha phase. The enamels based on gold and silver show that the tin site does not determine the colour of the enamel and the silver-gold association is likely to be the dominant factor. Mesoporous oxides show a link between the amount of mesoporous structure and their temperature stability. The nitrogen spectra of the template in the material shows that in the mesoporous silicate (which has the largest surface area) there is a breakdown of the amine into NH groups which does not appear in the other mesoporous materials. This could lead to a method of increasing the surface area of the other mesoporous oxides. QuadFit has the ability to simulate quadrupolar and CSA interactions with distributions of interactions whilst static and the quadrupolar interaction with distributions under MAS. The program is written in Java so will run on most platforms and also has near perfect stability

Structural studies on Functional Materials using Solid-State NMR, Powder X-ray Diffraction and DFT Calculations

Analytical and theoretical techniques were used in this work for structural studies of framework materials. One and two dimensional 31P and 17O solid state NMR experiments highlight subtle thermally induced structural changes in (MoO2)2P2O7 pyrophosphate, tungsten trioxide WO3 and negative thermal expansion ZrW2O8. DFT methods using CASTEP software to calculate 31P and 17O NMR parameters are performed on these structures and discussed in comparison to experimental results, published structures and thermal mechanisms