Alkaline-earth Stannates Research Articles

MgSnO3 epitaxial thin films for solar-blind photodetection: Fabrication and properties

In alkaline earth stannates, MgSnO3 has low density, high specific strength and wide bandgap (∼4.0 eV), which is a particularly promising semiconductor material. However, the lack of single crystal film limits the application of such materials in photoelectric devices. Here, high-quality MgSnO3 epitaxial film was grown on SrTiO3 substrate by pulsed laser deposition (PLD). The film grown under an oxygen pressure of 5 Pa at 900 °C presented the best crystal quality, which has high transmittance (>93 %) and wide bandgap (3.94 eV). The in-plane and out-of-plane epitaxial relationships between the film and substrate are MgSnO3[1 1‾ 00] ⎜⎜SrTiO3[1 1‾ 0] and MgSnO3(0001) ⎜⎜SrTiO3(111), respectively. Due to the lower formation energy of Mg vacancies compared to Sn vacancies, the Mg/Sn molar ratio in the film is less than 1, increasing with the oxygen pressure rising during the film. For the solar-blind photodetector based on 10 Pa sample, the device presents a photo-responsivity of 3.76 × 10−3 A ∙ W−1 and a fast speed of photoresponse (<0.05 s). The excellent performance exhibited by single crystal thin films will undoubtedly promote the development of MgSnO3 based single crystal devices.

Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO3.

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO3 has the highest measured band gap in this family and is thus a particularly promising ultrawide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had described this material as "undopable". Here we redeem CaSnO3 using hybrid molecular beam epitaxy, which provides an adsorption-controlled growth for the phase-pure, epitaxial, and stoichiometric CaSnO3 films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO3 with free electron concentrations, n3D, from 3.3 × 1019 cm-3 to 1.6 × 1020 cm-3. The films exhibit a maximum room-temperature mobility of 42 cm2 V-1 s-1 at n3D = 3.3 × 1019 cm-3. Despite having a comparable radius as the host ion, La expands the lattice parameter. Using density functional calculations, this effect is attributed to the energy gain by lowering the conduction band upon volume expansion. Finally, we exploit robust doping by fabricating CaSnO3-based field-effect transistors. The transistors show promise for CaSnO3's high-voltage capabilities by exhibiting low off-state leakage below 2 × 10-5 mA/mm at a drain-source voltage of 100 V and on-off ratios exceeding 106. This work serves as a starting point for future studies on the semiconducting properties of CaSnO3 and many devices that could benefit from CaSnO3's exceptionally wide band gap.

Tuning the catalytic performance of CaSnO3 by developing an S-scheme p–n heterojunction through Ag6Si2O7 doping

Perovskites, especially alkaline earth stannates, have drawn enormous interest from the research community owing to their low chemical toxicity, environmental friendliness, excellent optical properties, and good thermal and chemical stabilities.

Post-annealing optimization of the heteroepitaxial La-doped SrSnO3 integrated on silicon via ALD.

Wide band gap (WBG) alkaline-earth stannate transparent oxide semiconductors (TOSs) have attracted increasing attention in recent years for their high carrier mobility and outstanding optoelectronic properties, and have been applied widely in various devices, such as flat-panel displays. Most alkaline-earth stannates are grown by molecular beam epitaxy (MBE); there are some intractable issues with the tin source including the volatility with SnO and Sn sources and the decomposition of the SnO2 source. In contrast, atomic layer deposition (ALD) serves as an ideal technique for the growth of complex stannate perovskites with precise stoichiometry control and tunable thickness at the atomic scale. Herein, we report the La-SrSnO3/BaTiO3 perovskite heterostructure heterogeneously integrated on Si (001), which uses ALD-grown La-doped SrSnO3 (LSSO) as a channel material and MBE-grown BaTiO3 (BTO) as a dielectric material. The reflective high-energy electron diffraction and X-ray diffraction results indicate the crystallinity of each epitaxial layer with a full width at half maximum (FWHM) of 0.62°. In situ X-ray photoelectron spectroscopy results confirm that there was no Sn0 state in ALD-deposited LSSO. Besides, we report a strategy for the post-treatment of LSSO/BTO perovskite heterostructures by controlling the oxygen annealing temperature and time, with a maximum oxide capacitance Cox of 0.31 μF cm-2 and a minimum low-frequency dispersion for the devices with 7 h oxygen annealing at 400 °C. The enhancement of capacitance properties is primarily attributed to a decrease of oxygen vacancies in the films and interface defects in the heterostructure interfaces during an additional ex situ excess oxygen annealing. This work expands current optimization methods for reducing defects in epitaxial LSSO/BTO perovskite heterostructures and shows that excess oxygen annealing is a powerful tool for enhancing the capacitance properties of LSSO/BTO heterostructures.

Strain effect on the ground-state structure of Sr2SnO4 Ruddlesden-Popper oxides

Ruddlesden-Popper (RP) oxides (${A}_{n+1}{B}_{n}{\mathrm{O}}_{3n+1}$) comprised of perovskite ${(AB{\mathrm{O}}_{3})}_{n}$ slabs can host a wider variety of structural distortions than their perovskite counterparts. This makes an accurate structural determination of RP oxides more challenging. In this study, we investigate the structural phase diagram of $n=1$ RP ${\mathrm{Sr}}_{2}{\mathrm{SnO}}_{4}$, one of alkaline earth stannates that are promising for opto-electronic applications by using group theory based symmetry analysis and first-principles calculations. We explore the symmetry breaking effects of different dynamical instabilities, predict the energies of phases they lead to, and take into account different (biaxial strain and hydrostatic pressure) boundary conditions. We also address the effect of structural changes on the electronic structure and find that compressive biaxial strain drives ${\mathrm{Sr}}_{2}{\mathrm{SnO}}_{4}$ into a regime with wider band gap and lower electron effective mass.

Morphology and facet tailoring of CaSnO3 assembled in molten salt with defect-mediated photocatalytic activity

Modulation of perovskite oxide toward controlled active facets and defects has attracted much attention. However, the influence of facets and defects on the photocatalytic activity are still ambiguous, especially for the alkaline-earth stannates. In this work, a high-efficiency CaSnO3 semiconductor photocatalyst was prepared by a facile one-step molten salt method without adding a capping agent. By simply varying the solutes, three kinds of CaSnO3 with different morphology were successfully synthesized, including sphere, cube, and cuboctahedron. Meanwhile, both the facet exposure and formation of defects affected the photocatalytic performance of CaSnO3 samples. The formation mechanism for various CaSnO3 morphology and their facet-dependent photocatalytic performances were explored in detail, which indicated that the formation of {100} facets was beneficial for the photocatalytic performance rather than {111}. Although these samples showed similar absorption edges, the varying amount of oxygen vacancy was one of the reasons for the diverse photocatalytic activity. The distortion of crystal structure of CaSnO3 was due to the formation of reduced Sn, which could influence the bond and angle of Sn–O–Sn and subsequently the photocatalytic activity. Different scavengers were also used to identify the role of active species in the photo-degradation process. The relationships between surface electronic structure, oxygen vacancy, and oxidation state of B site with the photocatalytic activity of tin stannate were clearly revealed.

Precursor selection in hybrid molecular beam epitaxy of alkaline-earth stannates

One of the challenges of oxide molecular beam epitaxy (MBE) is the synthesis of oxides containing metals with high electronegativity (metals that are hard to oxidize). The use of reactive organometallic precursors can potentially address this issue. To investigate the formation of radicals in MBE, we explored three carefully chosen metal-organic precursors of tin for SnO2 and BaSnO3 growth: tetramethyltin (TMT), tetraethyltin (TET), and hexamethylditin (HMDT). All three precursors produced single-crystalline, atomically smooth, and epitaxial SnO2 (101) films on r-Al2O3 (101¯2) in the presence of oxygen plasma. The study of growth kinetics revealed reaction-limited and flux-limited regimes except for TET, which also exhibited a decrease in the deposition rate with increasing temperature above ∼800 °C. Contrary to these similarities, the performance of these precursors was dramatically different for BaSnO3 growth. TMT and TET were ineffective in supplying adequate tin, whereas HMDT yielded phase-pure, stoichiometric BaSnO3 films. Significantly, HMDT resulted in phase-pure and stoichiometric BaSnO3 films even without the use of an oxygen plasma (i.e., with molecular oxygen alone). These results are discussed using the ability of HMDT to form tin radicals and therefore assisting with Sn → Sn4+ oxidation reaction. Structural and electronic transport properties of films grown using HMDT with and without oxygen plasma are compared. This study provides guideline for the choice of precursors that will enable the synthesis of metal oxides containing hard-to-oxidize metals using reactive radicals in MBE.

Structure, Strain Relaxation and Defects in Alkaline Earth Stannates Films

Journal Article Structure, Strain Relaxation and Defects in Alkaline Earth Stannates Films Get access Bharat Jalan Bharat Jalan University of Minnesota, Minneapolis, Minnesota, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 610–611, https://doi.org/10.1017/S1431927620015275 Published: 01 August 2020

Engineering SrSnO3 Phases and Electron Mobility at Room Temperature Using Epitaxial Strain.

High-speed electronics require epitaxial films with exceptionally high carrier mobility at room temperature (RT). Alkaline-earth stannates with high RT mobility show outstanding prospects for oxide electronics operating at ambient temperatures. However, despite significant progress over the last few years, mobility in stannate films has been limited by dislocations because of the inability to grow fully coherent films. Here, we demonstrate the growth of coherent, strain-engineered phases of epitaxial SrSnO3 (SSO) films using a radical-based molecular beam epitaxy approach. Compressive strain stabilized the high-symmetry tetragonal phase of SSO at RT, which, in bulk, exists only at temperatures between 1062 and 1295 K. We achieved a mobility enhancement of over 300% in doped films compared with the low-temperature orthorhombic polymorph. Using comprehensive temperature-dependent synchrotron-based X-ray measurements, electronic transport, and first principles calculations, crystal and electronic structures of SSO films were investigated as a function of strain. We argue that strain-engineered films of stannate will enable high mobility oxide electronics operating at RT with the added advantage of being optically transparent.

Synthesis, Characterization, Optical and Transport Properties of BaSnO&amp;lt;SUB&amp;gt;3&amp;lt;/SUB&amp;gt; Synthesized by Wet Chemical Route

Alkaline earth stannates have renewed attention as potential materials for gas sensing, transparent conducting oxides and ceramic capacitor applications. A noble wet chemical route is chosen to synthesize BaSnO3 powders having micron size grains. Problems related to uneven grains and poor densification in conventional solid state route have been overcome in the present method. X-ray diffraction data confirms the single phase formation without any secondary phase having cubic perovskite structure (S.G. Pm-3m). The estimated lattice parameter is ≈4.11933A and percentage density is ≈90%. The microstructure probed by scanning electron micrograph reveals almost homogeneously distributed particles having random shape with some porosity. Phonon modes in FT-IR and Raman spectra confirm the local symmetry distortion. The DC conductivity at 200°C approaches almost 2 × 10-2 (Ω-cm)-1, which is almost of the same order as that observed in perovskite based electronic conductors. DC and a.c. electrical conductivity of the material follow Mott’s variable range hopping at low temperature and Arrhenius type thermally activated process at elevated temperatures. Evidence of small polar on formation is also verified. Optical band gap estimated from absorption data is ≈2.94 very close to the reported value. Composition prepared through the noble wet chemical route has high electrical conduction and suitable for gas sensing applications.

CaSnO 3 obtained by modified Pechini method applied in the photocatalytic degradation of an azo dye

Abstract Pure forms of alkaline-earth stannates with perovskite structure (ASnO3, A= Ca2+, Sr2+, Ba2+) have been used as photocatalysts. In this work, CaSnO3 perovskite sample was synthesized by a modified Pechini method at 800 ºC and characterized by X-ray diffraction (XRD), UV-visible spectroscopy, infrared spectroscopy and Raman spectroscopy. The photocatalytic degradation of remazol golden yellow (RNL) dye under UV radiation was evaluated. The XRD pattern showed that the synthesis method favored the orthorhombic CaSnO3 crystallization. According to the Raman spectrum, a material with high short-range order was obtained despite of the relatively low synthesis temperature, compared to the solid-state reaction one. The highest photocatalytic activity was attained at pH 3, which presented 51% discoloration and improved activity of 35% compared to discoloration solely due to adsorption (absence of radiation). The point of zero charge (PZC) and the photocatalytic results indicated that a direct mechanism prevailed at pH 3, whereas an indirect mechanism prevailed at pH 6.

Luminescence and Valence of Tb Ions in Alkaline Earth Stannates and Zirconates Examined by X-ray Absorption Fine Structures.

The difference in Tb3+ green luminescence intensities in doped perovskite(ABO3)-type alkaline earth stannates, AeSnO3 (Ae = Ca, Sr, Ba), and the Mg codoping effect on the luminescence intensities in doped CaMO3 (M = Sn, Zr) were investigated utilizing the X-ray absorption fine structures (XAFS) of the Tb LIII absorption edge. It is considered that the local symmetry at A sites is responsible for the different Tb3+ luminescence intensities in AeSnO3 (Ae = Ca, Sr, Ba) doped with Tb ions at A sites. However, it was found from the XAFS spectra that some Tb ions are unintentionally stabilized at B sites as Tb4+, especially in BaSnO3. Not only the central symmetry for Tb3+ at A sites but also the presence of Tb4+ at B sites were considered to bring about the absence of Tb3+ luminescence in doped cubic BaSnO3. No obvious changes in the Tb3+ local structure at A sites were detected between Tb single doped and Tb-Mg codoped CaMO3 (M = Sn, Zr) from the extended XAFS oscillation, but the trace of Tb4+ at B sites in the Tb single doped sample was observed in the X-ray absorption near edge structures. It is, therefore, considered that the Tb3+ luminescence enhancement by Mg codoping is primarily attributed to the charge compensation rather than the changes in the local structure around Tb3+ at A sites.

Cathodoluminescence in Rare Earth Doped Perovskite Films for RGB Colors

We have studied PL and EL properties of chemically stable perovskite phosphor films. Pr-doped Ca0.6Sr0.4TiO3 perovskite films were grown epitaxially on SrTiO3 (001) substrates and their intense red luminescence were observed. The PL intensity was markedly increased by post-annealing treatments, and the maximum PL intensity was obtained by the combination of relatively low-temperature film growth at 600 °C and sequential rapid post-annealing at 1000 °C. 1 We have discovered perovskite thin film electroluminescence, opening a new optical application of regarding perovskite materials. Complete epitaxial growth of all of the layers and very flat interfaces with monoatomic steps have been obtained. With increasing driving voltage, the intensity of electroluminescence dramatically increases. The sharp electroluminescence peak at around 610 nm at 12 V becomes much stronger with increasing ac voltage. High-quality red color is produced and the working voltage for whole-surface electroluminescence is as low as 10 V. 2 The electron beam irradiation was a method of obtaining electrical and optically emitting. In this report, we show preparation of cathodoluminescence in perovskite films for RGB colors. The films were grown on double-side polished SrTiO3 (001) substrates by pulsed laser deposition. The single-phase polycrystalline targets were prepared by a conventional solid-state reaction method. Compositions of green, blue and red were [(Ca0 . 97Mg0 . 03)0 . 98Tb0 . 02]SnO3, Sr2(Sn0.95Ti0.05)O4 and Pr0.002(Ca0.6Sr0.4)0.997TiO3, respectively. During film growth, the SrTiO3 substrates were heated to 600 °C for film and the oxygen partial pressure was controlled at 50 Pa. An ArF excimer laser (λ = 193 nm) was used that had a repetition rate of 8 Hz and a fluence of around 1.2 J cm−2 pulse−1 at the target surface. In order to take out luminescence excited by the electron beam from the side opposite to with a film adhesion side, the both-side polished substrate which has about 70% (at 500nm) of transparent was used. The field emitter with a gate electrode was used for the source of electrons. The transmittance of film with both-side polished substrate after 1000°C post-annealing treatments was 67 % (at 500nm), indicating that the transmittance of the epitaxially grown alkaline-earth stannates film is very high. When the phosphor film of [(Ca0 . 97Mg0 . 03)0 . 98Tb0 . 02]SnO3was irradiated with the electron beam, the cathode luminescence peaks were observed at around 490, 543, 582, 620 nm. These results clearly show that CL has been successfully obtained in the perovskite films. By using thin film phosphor for FED, high resolution could be realized and it is efficient and also possibility of a long-life. REFERENCES (1) H. Takashima, et al, Applied Physics Letters. 89 261915 (2006). (2) H. Takashima, et al, Advanced Materials . 21 3699 (2009)

Luminescence and Storage Properties of Sm-Doped Alkaline-Earth Atannates

The thermoluminescence, persistent luminescence and storage properties of Sm-doped alkaline-earth stannates, A2SnO4:Sm3+ (A = Ca, Sr, Ba), are studied above room temperature. Different characteristic features of the thermoluminescence glow curves of A2SnO4:Sm3+ are observed, which implicates A2SnO4:Sm3+ could provide different potential applications. Ca2SnO4:Sm3+ with suitable traps shows efficient persistent luminescence, the red persistent luminescence could be observed in the dark for about several hours. Sr2SnO4:Sm3+ possesses relative deep traps fulfill the requirement of a good storage phosphor, and the different amount of the stored energy could be released under the various treatments (the excitation with 980/680 nm light or heating at 200°C). The extremely weak PL intensity of Ba2SnO4:Sm3+ implies that it is inadequate applied as storage phosphor or afterglow phosphor. The decay time of afterglow increase in the order of Ba2SnO4:Sm3+ < Sr2SnO4:Sm3+ < Ca2SnO4:Sm3+. Our analysis indicates that the persistent luminescence properties of A2SnO4:Sm3+ are governed by the cation vacancy, serve as the hole trap, which accept one hole to get stabilized.

Crystallization study of SrSnO3:Fe

Alkaline earth stannates have recently become important materials in ceramic technology due to its application as humidity sensor. In this work, alkaline earth stannates doped with Fe 3? were synthesized by the poly- meric precursor method, with calcination at 300 C/7 h and between 400 and 1100 C/4 h. The powder precursors were characterized by TG/DTA after partial elimination of car- bon. Characterization after the second calcination step was done by X-ray diffraction, infrared spectroscopy, and UV- vis spectroscopy. Results confirmed the formation of the SrSnO3:Fe with orthorhombic perovskite structure, besides SrCO3 as secondary phase. Crystallization occurred at 600 C, being much lower than the crystallization tem- perature of perovskites synthesized by solid state reaction. The analysis of TG curves indicated that the phase crys- tallization was preceded by two thermal decomposition steps. Carbonate elimination occurred at two different temperatures, around 800 C and above 1000 C.

Novel porous CaSnO 3:Eu 3+ and Ca 2SnO 4:Eu 3+ phosphors by co-precipitation synthesis and postannealing approach: A general route to alkaline-earth stannates

Novel porous CaSnO 3:Eu 3+ and Ca 2SnO 4:Eu 3+ phosphors have been successfully prepared by postannealing the corresponding precursors at elevated temperatures. The precursors were firstly obtained by co-precipitation method at room temperature. The as-prepared phosphors were well characterized by means of X-ray powder diffraction (XRPD) and field emission scanning electron microscopy (FESEM). The photoluminescent excitation and emission spectra illustrate the red-emitting nature of CaSnO 3:Eu 3+ and Ca 2SnO 4:Eu 3+ phosphors. Moreover, CaSnO 3:Eu 3+ phosphors with various shapes were obtained by introducing certain kinds of additives. Importantly, this facile synthetic method can be readily extended to prepare many other alkaline-earth stannates including BaSnO 3:Eu 3+, Ba 2SnO 4:Eu 3+, SrSnO 3:Eu 3+, and Sr 2SnO 4:Eu 3+ phosphors.

Photoluminescence properties of Pr doped and Tb–Mg codoped CaSnO 3 with perovskite structure

In the survey of photoluminescence properties of rare-earth doped alkaline-earth stannates, CaSnO 3 was found to show intense white luminescence by Pr doping and green luminescence by Tb–Mg codoping. Pr doped CaSnO 3 showed not only a red emission line originating from the f–f transition from 1D 2 to 3H 4 state, which is observed in Pr doped CaTiO 3 or SrTiO 3, but also an intense blue–green line assignable to the f–f transition from 3P 0 to 3H 4 state in Pr 3+ ions. Because of the mixture of the red and blue–green luminescence, the Pr doped CaSnO 3 was seen in white under uv excitation. Tb–Mg codoped CaSnO 3 showed a sharp green emission line derived from the f–f transition of from 5D 4 to 7F 5 state in Tb 3+ ions along with other small emission lines. Tb–Mg codoping effectively enhanced the green luminescence of Tb 3+ ions probably due to the local lattice distortion around Tb ions induced by the small Mg ions at Ca sites. The photoluminescence properties of Pr doped and Tb–Mg codoped ASnO 3 (A = Sr, Ba) and CaMO 3 (M = Ti, Zr) with perovskite structures were also examined and compared to understand the luminescence mechanisms in these materials.

Blue photoluminescence in Ti-doped alkaline-earth stannates

Blue photoluminescence properties of Ti-doped alkaline-earth stannates, A 2(Sn 1− x Ti x )O 4 ( A=Ca, Sr, Ba) ( x=0.005–0.15), were examined at room temperature. These stannates showed intense broad emission bands peaking at 445 nm for Ca 2SnO 4, at 410 nm for Sr 2SnO 4, and at 425 nm for Ba 2SnO 4 under UV excitation. Emission intensities were relatively insensitive to Ti concentration and no sharp concentration quenching was observed. Mixing alkaline-earth ions in the crystal structures did not increase the emission intensities in the A 2(Sn 1− x Ti x )O 4 system. The excitation spectra of these stannates exhibited broad bands just below the fundamental absorption edges, implying that luminescence centers do not consist of the component elements in the host materials. It was suggested that the isolated TiO 6 complexes are possible luminescence centers in these materials, as previously proposed in other Ti-doped stannates such as Mg 2SnO 4 and Y 2Sn 2O 7.

Phase evolution and microstructural development in sol-gel derived MSnO3 (M = Ca, Sr and Ba)

Alkaline-earth stannates having the general chemical formula MSnO3 (M = Ca, Sr and Ba) have been projected as potential electronic ceramics. In view of the information gaps in the reported research, a vigorous and systematic investigation on these exotic materials has been carried out. In this communication, the synthesis of CaSnO3, SrSnO3 and BaSnO3 via sol-gel technique is reported. Infrared spectroscopy and X-ray analyses of various gel samples with different thermal history helped in identifying the reaction pathways and the stage where amorphous gel to crystalline phase transition occurred. Grains of submicron size with narrow size distribution and spherical morphology, were the most noticeable characteristics of sintered calcium metastannate derived by sol-gel method. In the case of barium analogue, a fascinating ‘sugar cube’ structure (akin to that observed in solid-state reaction and the self-heat-sustained reaction derived samples) having improved density characteristics evolved at low sintering temperatures. This gradually transformed into a more familiar spherical granular motif with improved density characteristics as the sintering profiles were varied from 1200 °C/24 h to 1500 °C/2 h. This seems to be an inherent feature of this system, irrespective of the method of synthesis.

Preparation and characterization of barium stannate BaSnO3

Stannates have received much attention in recent years as components of ceramic dielectric bodies [1, 2]. Stannates of the general formula MSnO3, where M Ca, Sr or Ba, form a component of dielectric materials used for thermally stable capacitors [3, 4]. Barium stannate has found increasing applications in dielectric devices as a constituent of complex pervoskite solid solutions. Wainer [5, 6] disclosed devices comprised of the alkaline earth stannates and alkaline earth titanates in a series of patents. Recently it has been reported that thin films of BaSnO3 can be used for sensing nitrogen oxide and carbon monoxide gases [7, 8]. Unlike alkaline earth titanates MTiO3 (M Sr, Ca, Ba and Pb), these stannates have not been studied extensively in pure and doped form. For the last few years, we have been synthesizing and characterizing these stannates in doped and undoped forms [9‐13]. The results of these investigations prompted us to synthesize and characterize undoped BaSnO3. Barium stannate was prepared by the solid state ceramic route, using BaCO3 and SnO2 having purity 99.5% and 99.99%, respectively. Stoichiometric amounts of BaCO3 and SnO2 were mixed in a ball mill for 6 h using acetone as the mixing media. The mixed powder was dried in an oven overnight. The dry mixed powder was calcined in a platinum crucible at 1475 K for 6 h in air. The calcined powder was ground, mixed and pelletized (a die of 12 mm diameter was used for 2‐3 mm thick pellets) at various loads starting from 30 kN to 70 kN at intervals of 10 kN load. This was done to obtain an optimum load for the maximum green density. The