NEW TRIPLE AND QUADRUPLE PEROVSKITE OXIDES: SYNTHESIS, STRUCTURE AND PROPERTIES

Abstract

Transition metal oxides continue to be at the forefront of materials chemistry research due to their numerous structures and diverse physicochemical properties. The compositions one can think of for mixed metal oxides are seemingly innumerable. However, the structures that are adopted mainly controlled by their compositions and type of metal ion combinations that make them. Depending on the chemical compositions, the envisaged compounds may adopt various structure types, such as, rock salt, spinel, ilmenite, pyrochlore, perovskites and so forth. Perovskites are a noteworthy class of compounds to show exciting and often unusual physical and chemical properties that are of technological importance. For example, superconducting, giant/colossal magnetoresistive, ferromagnetic, high-k dielectric, half-metallic, multiferroic, catalytic and so on are notable among various properties exhibited by perovskites. These properties are produced by the combination of different metals/non-metals at the A- and B-sites of the ABO3 perovskites and their derivatives. The perovskites with the general ABO3 composition and based on corner connected polyhedral network (mainly octahedral) can broadly be classified into four types, namely (i) Simple perovskites, represented by ABO3, (ii) Double perovskites, represented by A2B2O6 (also A2BB´O6, AA´B2O6, AA´BB´O6 etc.), (iii) Triple perovskites, represented by A3B3O9 (also A3B2B´O9, A3BB´B´´O9 etc.), and (iv) Quadruple perovskites, represented by A4B4O12 (also AA´3B4O12, AA´3B2B´2O12 etc.). Although, the name ‘perovskite’ was given by the German mineralogist Gustav Rose in the honor of Russian mineralogist Count Lev Aleksevich Von Perovski (1792–1856) back in nineteenth century typically to identify the compound ‘CaTiO3’, it has become so familiar to the scientific community with the latest developments of perovskite solar cells. The general formula of the perovskites are ABX3, where both A and B are two cations of different ionic radii and X is an anion that coordinate to both. X is mostly oxygen but other ions, such as halides, nitrides and sulfides are also possible. In the perovskite family, the double perovskites are one of the most investigated class of compounds, where numerous research groups have contributed to a large body of research across the globe. Structurally, the double perovskites are also diverse. They show interesting B-site cation ordering, such as, rock salt, columnar and layered. Though rare, A-site rock-salt cation ordering is also known. The connectivity of octahedra in triple perovskite compositions may either be exclusively through corners or both corner and edge shared. For example, Sr3CaRu2O9 and Sr3Fe2TeO9 shows corner connected 1:2 or 2:1 B-site ordering of Ca and Ru or Fe and Te cations, respectively, whereas Ba3GaTiSbO9 shows corner as well as edge shared connectivity. In the latter case, different types structural variations with edge and corner connectivity gives rise to the so-called polytypic structures exhibiting interesting magnetic and electronic properties. Oxides of compositions, such as AA´3B4O12, AA´A´´2B4O12 or AA´3B2B´2O12 and many others may adopt the quadruple perovskite structure. For example, CaCu3Ti4O12, CaCu3Fe4O12, LaCu3Fe4O12, BiMn3Mn4O12 CaCu3Ru4O12, CaCu3Rh4O12, LaMn3Rh4O12, CaCu3Fe2Nb2O12, CaCu3Fe2Sb2O12, CaCu3Fe2Re2O12, LaCu3Mn3FeO12, Sm2MnMn(Mn4-xTix)O12 (1 ≤ x ≤ 3) and so on are known to adopt the quadruple perovskite structure. Stabilization of various compositions and structures based on the basic perovskite and its derived frameworks have given rise to the occurrence of diverse physicochemical properties, such as, magnetic, dielectric, magnetoresistive, photovoltaic, luminescent and catalytic, to illustrate a few. A brief overview of the perovskites and their variants is given in Chapter-1. The interest in quadruple perovskites have grown in recent times. This mainly stems from couple of reasons. In contrary to other 3D perovskites including those of simple, double and hexagonal ones, the quadruple perovskites contain smaller sized transition metals at their A-sites, thus incorporating additional A/AOB/B electronic interactions over already existent BOB interactions of regular perovskites. Moreover, the ordering of ions at the A- and B-site can influence the physical properties to large extents thereby can lead toward unprecedented physical phenomenon in these oxides. Lastly, most of the quadruple perovskites that show high ferri-/ferromagnetic transition temperatures are formed mostly at high temperature and high-pressure condition. The requirement of high-pressures up to about 20 GPa limits the amount of samples that can be synthesized in a reaction. For feasible applications, bulk synthesis of high transition temperature quadruple perovskites under ambient pressure is highly desirable. Moreover, added dielectric properties may incorporate additional multifunctional property in these oxides. In the light of the above, the bulk synthesis of new triple and quadruple perovskite compositions under ambient pressure is envisaged in the present investigation. As the conventional solid-state synthesis continue to be one of the most exploited method for the exploratory synthesis of new transition metal oxides, we have used the same in the present thesis. For this purpose, the starting materials used for the synthesis are largely high purity metal oxides, carbonates or oxalates. Initially, the solid-sate reactions are carried out on exploratory basis by heating the stoichiometric mixture of reactants at different temperatures for various durations. The product mixtures obtained at each step are analyzed by routine powder X-ray diffraction (P-XRD) study and conditions for the formation of single-phase powder products are ascertained. Then the synthesized compounds are further characterized by the various techniques, such as, slow-scan P-XRD, field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), magnetometry (using superconducting quantum interference device magnetometer, SQUID/ vibrating sample magnetometer, VSM), electrical resistivity, heat capacity and impedance analysis. The details of synthesis, the techniques and instruments employed are briefly described in Chapter-2. Chapter-3 describes the synthesis, structure, magnetic and dielectric properties of new triple perovskite compositions, A3MTiSbO9 (A = Sr, Ba; M = Mn, Co). All the compounds are prepared by conventional solid-state reaction method. The compounds are characterized by P-XRD, FE-SEM, EDS, HR-TEM, XPS, SQUID magnetometry and dielectric measurements. While the Sr-containing compounds form in cubic 3C perovskite structures, the Ba analogues adopt the hexagonal 6H perovskite structure. Rietveld refinements are carried out using P-XRD data in Pm3̅m and P63/mmc space groups for cubic and hexagonal perovskites, respectively. A complete cation disorder at the B-sites for the 3C phases, Sr3MnTiSbO9 and Sr3CoTiSbO9, and ~ 20% cation disorder at the 4f sites for the hexagonal phases, Ba3MnTiSbO9 and Ba3CoTiSbO9, are evident in the Rietveld structure refinements. The effective paramagnetic moments observed for the compounds are in support of 3+ oxidation states for Mn and Co, in conformity with the XPS result and respective cation disorders present in the compounds. While the 3C perovskites show magnetic anomalies at ~ 10 and 20 K, respectively, the hexagonal perovskites reveal paramagnetic behaviour up to 5 K. The observed magnetic anomalies for the cubic perovskites are attributed to short-range antiferromagnetic correlations suggestive of a cluster-glass type behaviour. The magnetic moment values supports a HS Co3+ configuration at the 2a sites while maintaining a LS Co3+ at the 4f sites for Ba3CoTiSbO9. The higher dielectric permittivity of cubic phases, as compared to the hexagonal phases, is ascribed to the presence of higher space charge polarizations at lower frequencies. In Chapter-4, the ambient pressure synthesis, structure, magnetic and transport properties of a series of new A-site ordered quadruple perovskites, LnCu3Mn1+xTi3-xO12 (Ln = La, Nd; x = 0, 0.3) are reported. Rietveld structure refinements using the P-XRD data confirmed the formation of A-site ordered quadruple perovskites for all the compositions in the body centered cubic symmetry (space group Im3̅ , No. 204). FE-SEM images show cubic shaped particles with particle edges rounded off in the entire the region of imaging, while the EDS data confirmed the elemental ratio in agreement with the nominal compositions. XPS analysis indicate the presence of Cu in 2+ in all cases, while Mn is present in 3+ state for x = 0 and in a mixed valent state (both 3+ and 4+) for x = 0.3 compositions. While LaCu3MnTi3O12 and LaCu3Mn1.3Ti2.7O12 show onset of ferrimagnetic order at ~ 60 and 80 K, respectively, NdCu3MnTi3O12 and NdCu3Mn1.3Ti2.7O12 show similar behavior at ~ 110 and ~140 K, respectively. It is interesting to note that the onset of ferrimagnetic ordering temperature is shifted to higher temperatures with the replacement of La by Nd and with the introduction of mixed valency in Mn (Mn3+ & Mn4+). It is believed that the strategy may be useful for the enhancement of TC in other quadruple perovskites. To understand the low-temperature magnetic behavior and exact magnetic structure of the compounds variable temperature neutron diffraction study is necessary. Electrical resistivity measurements show semiconducting behavior down to the lowest measuring temperatures (~ 125  175 K) for all the compounds. Ambient pressure synthesis, structure, magnetic and dielectric properties of a new A-site ordered quadruple perovskite oxide, LaCu3Fe2TiSbO12, is discussed in Chapter-5. The compound is prepared by solid-state reaction method at ambient pressure. Rietveld refinement of the crystal structure using P-XRD data reveal that LaCu3Fe2TiSbO12 forms in a body centered cubic symmetry with Im3̅ space group (No. 204). X-ray photoelectron spectroscopy indicates that the Cu and Fe valances are 2+ and 3+, respectively. The compound show ferrimagnetic ordering at ~ 60 K. Further studies are required to understand the origin of low-temperature antiferromagnetic-like transition and the divergence of FC and ZFC susceptibilities below ~ 25 K. However, the magnetization (M) vs. field (H) plot indicate hysteresis loop not only at 5 and 25 K with higher saturation magnetizations, but at 300 K as well though with lower magnetization values, indicating presence of ferri-/ferromagnetic correlations even at room temperature. The presence of a smeared hump and absence of a clear lambda-type transition in the specific heat data is indicative of a short-range magnetic order at ~ 160 K. Dielectric measurement reveal high dielectric constant (over 1200) and low dielectric loss for the compound. Temperature and field dependent dielectric studies may unravel any magneto-dielectric coupling or multiferroicity that may exist in this compound. Chapter-6 describes the ambient pressure synthesis of a new A- and B-site ordered quadruple perovskite, LaCu3Fe2RuSbO12, its structure and magnetic and transport properties. The compound is synthesized by conventional solid-state method. The composition has resulted by mere substitution of Ru in place of Ti in LaCu3Fe2TiSbO12, described in the previous chapter. While LaCu3Fe2TiSbO12 is a B-site disordered quadruple perovskite, replacement of Ti by Ru resulted in the stabilization of both A- and B-site ordered quadruple perovskite structure for LaCu3Fe2RuSbO12. Rietveld refinement reveals that the compound crystallizes in Pn3̅ space group. Magnetic studies reveal ferrimagnetic ordering at ~ 170 K. The M-H data show hysteretic behavior at room temperature (300 K) and even at 350 K, indicating short-range ferri-/ferromagnetic correlations above room temperature. The sharp rise in magnetization below 170 K and a clear kink (~ 170 K) in the specific heat data near the same temperature is indicative of the ferrimagnetic order. The saturation magnetization of the compound at 5 K is 4.18 μB / f.u., far more higher than that of LaCu3Fe2TiSbO12 (~ 1.25 μB / f.u.) but lower than expected for ferromagnetic order indicating that the compound is a ferrimagnet. Further studies are required to understand the origin of low-temperature antiferromagnetic-like transition and the divergence of FC and ZFC susceptibilities below ~ 70 K. To understand the low-temperature magnetic behavior and exact magnetic structure of the compound, variable temperature neutron diffraction study is necessary. Electrical transport data show semiconducting behavior according to variable range hopping mechanism and absence of any M-I transition. It is interesting to note that LaCu3Fe2RuSbO12 show much lower room temperature resistivity, higher electron localization length, longer NN hopping distance and much lower activation energy (Ea) as compared to LnCu3Mn1+xTi3-xO12 (Ln = La, Nd; x = 0, 0.3) perovskites. Chapter-7 presents the overall conclusions and future prospects of our current investigation. The present work have demonstrated composition dependent formation of cubic and hexagonal perovskites driven by Goldschmidt’s tolerance with the report of four new triple perovskite compositions, A3MTiSbO9 (A = Sr, Ba; M = Mn, Co). The compounds exhibited interesting interplay of magnetic and dielectric properties due to their 3C and 6H structures and cation order/disorder at the B-site of the perovskites. Subsequently, ambient pressure bulk synthesis of several A- and B-site ordered quadruple perovskites are demonstrated in the present study. The compounds are synthesized by conventional solid-state reactions. Rietveld structure refinements helped in the elucidation of the extent of A- and B-cation order in the compounds. Magnetic, dielectric and transport properties of the compounds are thoroughly investigated. Nd incorporation in La quadruple perovskites lead to an enhancement of the onset of ferrimagnetic ordering temperature of the A-site ordered quadruple perovskites. Similarly, incorporation of Mn mixed valency in the quadruple perovskite compositions also lead to an increase in the onset of ferrimagnetic ordering temperature in the compounds. A heterovalent coupled cation substitution in CaCu3Fe2Sb2O12 with the replacement of Ca by La and one of the Sb by Ti has resulted in the formation of new quadruple perovskite, LaCu3Fe2TiSbO12, with high dielectric permittivity. However, there is a decrease in the ferrimagnetic ordering temperature in the compound with respect to CaCu3Fe2Sb2O12. This is attributed to a disordered B-cation arrangement instead of an ordered cation arrangement as in the case of CaCu3Fe2Sb2O12. Further, incorporation of Ru in place of Ti in LaCu3Fe2TiSbO12 has induced B-site cation order in the quadruple perovskite and enhanced the ferrimagnetic ordering in the oxide significantly. It will be interesting to investigate the compounds by variable temperature neutron diffraction to fully understand the low-temperature magnetic transitions, ascertain the exact magnetic structures of the compounds, and unravel the implications of chemical manipulations for further enhancement of ferrimagnetic ordering above room temperature. Moreover, temperature and field dependent dielectric permittivity studies will be useful to unravel any magneto-dielectric coupling or multiferroic property that may exist in these compounds. Given that the simple perovskites have demonstrated good catalytic activity and the presence of multiple metal ions with both acidic and basic properties in a single-phase quadruple perovskite here, the compounds may be tested for their potential catalytic activities. Finally, there remains a large possibility to explore numerous chemical compositions with exotic physicochemical properties in this class of compounds.