Studies of the vibrational and magnetic properties of layered compounds

Abstract

In this final and concluding chapter, we summarize the important observations presented in the previous chapters and discuss the scope of future work in the systems under study. The main points dealt with in this thesis are: (i) Evolution of structure of solid solutions Fe???Ni?PS? as x goes from FePS? to NiPS?. (ii) Changes in the infrared bands due to small structural changes, which reflect the change in relative strengths of interlayer and intralayer interactions. (iii) Nature of magnetic transitions in the mixed antiferromagnetic systems Fe???Ni?PS?. Several important observations were made during the course of investigation, though we could not provide explanations for all of them. We summarize them below: The lattice contracts on going from FePS? to NiPS?. The most dominant change in the structure comes from the (M-S) distance (d???) and not from the (P-P) distance as stated in previous works. All the compounds are essentially ionic with some covalent character. NiPS? is more covalent than FePS?, as supported by Mössbauer results. Importantly, our structural analysis showed that there is preferential contraction along the a and b axes (in the a-b plane) compared to that along the c-axis when Fe is replaced by Ni. This makes the vibrational spectra of NiPS? more layer-like (more two-dimensional in nature). The high-frequency bands in the IR spectra of the solid solutions at room temperature arise from intralayer vibrations. The variation of frequency as a function of composition is very small. There are some interesting features observed in our IR spectra at the low-frequency region (<300 cm?¹). We observe splitting of the bands in NiPS? in the region (195 cm?¹ and 150 cm?¹) and also a doublet around (279 cm?¹-302 cm?¹). These split bands are explained in terms of correlation splitting arising from enhanced intralayer force constants due to preferential contraction of the a-b plane in NiPS?. The split bands (195 cm?¹ and 150 cm?¹) due to lattice contraction are similar to the low-temperature split bands of FePS? (lattice contraction due to anharmonicity). Some observed features remain unexplained. It is interesting to note that small changes in the structure can bring about significant changes in vibrational properties. The problem of mixed antiferromagnets is dealt with in detail in Chapter 6. The (?-T) curves give a wealth of information regarding the nature of magnetic transitions. The high-temperature series calculations done for NiPS? match very well and show that NiPS? is an almost isotropic Heisenberg system with small anisotropy, which ultimately drives the AFM transition. FePS? is essentially an Ising system with strong anisotropy. The mixed systems Fe???Ni?PS? undergo long-range AFM order throughout the composition range. There is no spin-glass-like ordering. Interestingly, up to x = 0.8, the mixed systems show behavior similar to the parent compound FePS?, whereas the Ni-rich compounds show behavior similar to NiPS?. We suggest that FePS? imposes its strong anisotropy on NiPS?, which has weak anisotropy. Hence, the mixed systems order like FePS? up to x = 0.8. In these essentially two-dimensional systems, the difference (T??? - T?) gives a qualitative estimate of the existence of short-range correlation. We find that even up to x = 0.9, (T??? - T?) < k, implying a sharp transition. This observation suggests that the mixed system orders more like an Ising system over an extensive concentration range. The strong anisotropy of one component ensures long-range order. In this thesis, we have presented a systematic study of structural, vibrational, and magnetic properties of the solid solutions Fe???Ni?PS? (0 ? x ? 1). To our knowledge, such a study on these layered compounds, which are also mixed antiferromagnets, has not been done before. There are many interesting and intriguing observations made, which we have tried to explain to the extent possible. In some places, our analysis may be qualitative and sketchy, but we have tried to present the essential physical picture. We should point out that both in Chapters 5 and 6, our observations would have been more complete if we were able to grow single crystals of mixed systems. Despite repeated attempts, we were not able to grow good single crystals of mixed systems. In the presence of single-crystal data, some of our conclusions (which we are now making with caution) could have been made more definite. Some of the experiments which we wanted to do but could not due to lack of time during the course of investigation are listed below: (i) Specific heat near AFM transition (ii) Raman spectroscopy to complement IR spectroscopy (iii) Magnetic data at high fields to look for spin-flop transition We propose these experiments as suggestions for future work.