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Register a new userTwo step solid state synthesis and Synchrotron X-ray characterizations of ceramic Co3TeO6; an improper multiferroic
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Added by
Harishchandra Singh Mr.
Publication date
2015
fields
Physics
and research's language is
English
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A two step solid state reaction route has been presented to synthesize monophasic cobalt tellurate (Co3TeO6, CTO) using Co3O4 and TeO2 as starting reagents. During synthesis, initial ingredient Co3O4 is found better than CoO in circumventing the intermediate Co5TeO8 or CoTeO3 phases. High resolution Synchrotron X-ray Diffraction has been used to probe different phases present in synthesized CTO and to achieve its single phase. Further, XANES studies near Co K and Te L-edge reveal mixed oxidation states of Co (i.e. Co2+ and Co3+) and +VI valence state of Te respectively, which is also confirmed with XPS. Charge imbalance due to different oxidation states of the Co-ions has been observed to be compensated by plausible Te-cations vacancy. Enhanced multiferroic properties like effective magnetic moment (JAP 116, (2014)) have been correlated with the present synthesis route.
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Ternary nitride materials hold promise for many optical, electronic, and refractory applications yet their preparation via solid-state synthesis remains challenging. Often, high pressures or reactive gasses are used to manipulate the effective chemical potential of nitrogen, yet these strategies require specialized equipment. Here we report on a simple two-step synthesis using ion-exchange reactions that yield rocksalt-derived MgZrN$_2$ and Mg$_2$NbN$_3$, as well as layered MgMoN$_2$. All three compounds show nearly temperature-independent and weak paramagnetic responses to an applied magnetic field at cryogenic temperatures indicating phase pure products. The key to synthesizing these ternary materials is an initial low-temperature step (300-450 $^{circ}$C) to promote Mg-M-N bond formation. Then the products are annealed (800-900 $^{circ}$C) to increase crystalline domains of the ternary product. Calorimetry experiments reveal that initial reaction temperatures are determined by phase transitions of reaction precursors, whereas heating directly to high temperatures results in decomposition. These two-step reactions provide a rational guide to material discovery of other bulk ternary nitrides.
We report observation of magneto-electric and magneto-dielectric couplings in ceramic Co3TeO6. Temperature dependent DC magnetization and dielectric constant measurements together indicate coupling between magnetic order and electronic polarization. Strong anomaly in dielectric constant at ~ 18K in zero magnetic field indicates presence of spontaneous polarization. Observations like weak ferromagnetic order at lower temperature, field and temperature dependences of the ferroelectric transition provide experimental verification of the recent theoretical proposal by P. Toledano et al., Phys. Rev. B 85, 214439 (2012). We provide direct evidence of spin-phonon coupling as possible origin of magnetic order.
Solid-state synthesis from powder precursors is the primary processing route to advanced multicomponent ceramic materials. Designing ceramic synthesis routes is usually a laborious, trial-and-error process, as heterogeneous mixtures of powder precursors often evolve through a complicated series of reaction intermediates. Here, we show that phase evolution from multiple precursors can be modeled as a sequence of pairwise interfacial reactions, with thermodynamic driving forces that can be efficiently calculated using ab initio methods. Using the synthesis of the classic high-temperature superconductor YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) as a representative system, we rationalize how replacing the common BaCO$_3$ precursor with BaO$_2$ redirects phase evolution through a kinetically-facile pathway. Our model is validated from in situ X-ray diffraction and in situ microscopy observations, which show rapid YBCO formation from BaO$_2$ in only 30 minutes. By combining thermodynamic modeling with in situ characterization, we introduce a new computable framework to interpret and ultimately design synthesis pathways to complex ceramic materials.
In the quest of understanding significant variations in the physical, chemical and electronic properties of the novel functional materials, low temperature Synchrotron X-ray Diffraction (LT-SXRD) measurements on CTO (a type-II) and CMTO (a type-I) multiferroics are presented. Magnetic phase diagram of CTO shows multiple magnetic transitions at zero fields, whereas, in CMTO, 20 K enhancement in the antiferromagnetic transition temperature is observed followed by near room temperature Griffiths phase. Rietveld analysis on LT-SXRD data of both the samples indicates important observations. For both CTO and CMTO, the magnetic anomalies are followed by structural anomalies, which is a clear signature of spin lattice coupling and the positive shift of spin lattice coupling from CTO to CMTO.
Evidence of coexistence of Co3+ with Co2+ in ceramic Co3TeO6 through XANES, DC magnetization and first principal studies is provided. XANES along with linear combination fit provide relative concentrations of Co2+ and Co3+.Temperature dependent DC magnetization exhibits the same antiferromagnetic behavior as observed in single crystal. The presence of both Co2+ and Co3+ suggests that if the later is in high spin state, the effective magnetic moment is similar to that observed in single crystal studies. In contrast, if Co3+ is in low spin state effective magnetic moment is similar to that observed in Co3O4. It is further shown that both Co2+ and Co3+ in high spin states constitute a favorable ground state through first principle calculations where Rietveld refined Synchrotron X-ray diffraction data are inputs.
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