The dielectric properties of single crystals and ceramics of BizV05 5 have been investigated as a function of pressure up to 4 GPa at room temperature.The dielectric constant and the loss are found to decrease with increase in pressure for all the samples under study. However. the rate of decrease of the dielectric constant in the case of partially grain oriented ceramic is higher than that of the single crystal and conventionally synthesised ceramic. The decrease in dielectric constant with increase in pressure is attributed to the decrease in ionic polarisation as a consequence of the decrease in interionic distances

Prasad, KVR

Ramasesha, Sheela K

Shankar, MV

Varma, KBR

NAL-IR

The dielectric properties of ferroelectric bismuth vandate,Bi2 VO5.5 under pressure

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Advances tn Hagh Pressure Sczence B Technology Edated by A.K.Sangh Tata McGraw-Hill, New Delhi The Dielectric Properties of Ferroelectric Bismuth Vanadate, Bi2V05.5 under Pressure Shrda K.  Raniasrsha', K. V.RPrasad, M.V.Shankar and I<.B.R.Varma Materials Research Centre, Indian Institute of Science, Bangalore-560 012 * Materials Science Division. National Aerospace Laboratories, Bangalore-,560 017 Abstract The dielectric properties of single crystals and ceramics of BizV05 5 have been investigated as a function of pressure up to 4 GPa at room temperature. The dielectric constant and the loss are found to decrease with increase in pressure for all the samples under study. However. the rate of decrease of the dielectric constant in the case of partially grain oriented ceramic is higher than tha t  of the single crystal and conventionally synthesised ceramic. The decrease in dielectric constant with increase in pressure is attributed to the decrease in ionic polarisation as a consequence of the decrease in interionic distances. 1 Introduction The dielectric and ferroelectric properties of a large number of crystals belonging to the perovskite family have been investigated under pressure I. The most striking effect of pressure on ferroelectric properties, from the thermodynamic point of view, is the shift in the curie temperature (Tc). The T, decreases with increasing pres- sure for displacive type of ferroelectrics '. For hydrogen honded crystals in which the ferroelectric-paraelectric transition is understood to be mixed type involving the order-disorder transition of the proton system to which the displacements of the ot,her ions are strongly coupled, the T, decreases with increasing pressure. The T, increases with increasing pressure in the case of materials in which the ferroelectric- paraelectric transition is believed to be completely order-disorder type. Yet another r lass  of ferroelectric oxides belonging to the family of multilayered interstitial com- pounds, commonly known as Aurivillius family of oxides with the general formula 86 Ramasesha et al [BizO2]’+ [A,_lB,O~,+l]z- are also interesting from the structural as well as ferro- electric properties view point ’? where the metal A is in 12 co-ordination (Ca, Sr, Ba etc) and B is in 6 co-ordination (Ti, Nb, Ta et,r). Though, the chemicai formula appears to be complicated, the structural scheme of these compounds is quite sim- ple. For instance, the n = 2 member consists of alternation of a layer of [Bi2O2Izt with two perovskite like layers of oxygen octahedra ‘. Recent investigations into the Biz03-Vz05 binary system has yielded a novel phase Bi2V05.5. This compound has been reported to be ferroelectric at room-temperature 5,6,7. Structural studies carried out on both polycrystalline and single crystalline materials suggest that it is the vanadium analogue of an n = 1 member of the above mentioned family of oxides and belonging to a non-centrosymmetric polar orthorhomhic system (mm2) with lattice parameters a = 16.573(4), b = 5.605(1) and c = 15.284(3) A. We have been investigating the dielectric, pyroelectric and ferroelectric properties of ceramics as well as single crystals of Bi2V05.5 as a function of temperature ’. X-ray powder diffraction studies carried out, as a function of temperature, indicate that bismuth vanadate undergoes two major phase transitions, of the kind mentioned below, at 725K and 827K lo: 725 K Non centrosymmetric Orthorhombic ( mm2) ==+ Ceutrosymrnetric 827 K Orthorhombic (mmm ) ===+ Centrosymmetric Tetragonal ( 4/mmm) We thought that it is worth examining the dielectric properties of BizV05.5 as a function of pressure in the light of the aforesaid phase transitions, since applying pressure is a more efficient way to alter the inter-atomic distances and also t,o in- duce other phase transitions that are not observed at atmospheric pressure. Though, uniaxial pressure studies will be more interesting particularly on the layered com- pounds, the present studies are made with quasi-hydrostatic pressure, since this al- lows much higher pressure to be applied without destroying the crystals. We report in this paper, the preliminary results obtained on the studies concerning the effect of quasi-hydrostatic pressure on the dielectric properties of BizV05 ceramics and single crystals at room temperature (300 K).  Dielectric properties of Bi2V05.5 under pressure 87 2 Experimental Polycryst,alline powders of required Bi2V05.5 were prepared by heating a stoichio- metric mixture of Bi20, and V z 0 5  initially upto about 775 K and then at 1070 K for about 24 hours with intermediate grinding st,eps. The powders were compacted and subsequently sintered at 1070 K for about 24 hours. The density of the sintered disc is 7.30 gm/cm3. Ceramics with partial grain orientation were prepared by quenching the melts of prereacted Bi2V05.5. The ceramics were annealed at 1070 K for about 8 hours, before carrying out any physical property studies. The density of these ceramics is 7.50 gm/c,m3. The single crystals employed for the present studies were grown by slow cooling technique. The growth was carried out using a muffle furnace associated with a pro- grammable temperature controller. The prereacted powder of Bi2V05.5 was packed into a platinum crucible of capacity 100 cc and heated to about 1400 K at the rate of 100 K/hr. It was soaked at this temperature for about 6 hours and cooled at a programmable rate of 5 K/hr down to 900 K. Subsequently it was cooled at  the rate of 20 K/hr to 600 K and then the furnace was~switched off. The temperature profile of the furnace is shown in Fig. 1. The crystallised melt was found to contain fine shiny platelets embedded in it. The crystals separated mechanically ( 4mm x 4mm x 0.5mm) were greyish black with layered habit. The single crystalline nature of the grown crystals was confirmed by the X-ray diffraction studies using CAD4 automatic four circle diffractometer (MoKcu radiation). The dielectric constant and the loss measurements were carried out at IOOkHz using Keithley LCZ meter (model 3330) as a function of pressure using a pair of tungsten-carbide opposed anvils with 12.5 dia anvil face. 1.5 x 1.5mm2 face and 0.2 mm thick sample was set in a 0.2 mm thick metal gasket (5mm i.d.) with talc and epoxy mixture such that the sample surface was fully exposed on both sides. After allowing the epoxy to set, talc- epoxy disc with the sample was separated from the metal gasket. A pyrophyllite gasket (5 mm i.d. ; 12.5mm 0.d. and 0.42 mm thick) was stuck on the anvil along with a 0.28mm thick talc disc inside it, which formed the lower gasket. This gasket contains a copper lead and a bismuth sample along with two leads for in-situ pressure calihration. The talc-epoxy disc with the sample was placed on top of the talc disc in the lower gasket. Another pyrophyllite gasket with the same dimension and also containing a 0.28 mm thick talc disc at the centre 88 Ramasesha et al along with a copper lead was placed on top of the lower gasket such that the sample was sandwiched between the copper leads. Aluminium foils cut exactly to the size of the samples were placed between the copper leads and the sample on both sides. 3 Results and discussion The X-ray powder diffraction pattern obtained for the powdered sample heated at 1070 K confirms the formation of the required BizVOss phase (Fig. 2a). The X-ray diffraction pattern recorded for the ceramic obtained by quenching the melt suggests that it is polycrystalline (Fig. 2b). However, the intensities of the (001) reflections ( Vit. (002)  and (006) ) are higher than those of the ceramics obtained by conventional solid state reaction route. It implies that a significant number of grains 1500 1100 - Y, + 700 Fig. 1 60Whr \  I I 300 0 40 80 120 TIME ( hr ) 10 20 30 40 50 60 2 e " ( C u K , )  Figure 1: The temperature profile employed to grow single crystals of bismuth vana- date. Figure 2: X-ray dzffmction patterns f o r  the (a) conventionally fired ceramic and ( b )  crramic obtained by melt-quenching, of the ceramics obtained by quenching the melt is oriented along the c- direction. The degree of the grain orientation is determined based on the X-ray orientation method Dielectric properties of BizV05.5 under pressure 89 proposed by Lotgering ll. It is found that 55% of the grains are c-oriented, The scanning electron micrographs of the ceramics obtained by the conventional solid state reaction route and by quenching the melts are shown in Fig. 3a and b respectively. A rectangular bar shaped grain morphology is found with the ceramic prepared by melt-quenching technique. The average grain size as-determined by the line intercept method is about 100 pm x 35 pm. The growth rate of the individual grains in the plane normal to the pressing direction appears to be high. Figure 3: Scanning electron micrographs of unetched (a) conventionally fired ceramic and ( b )  ceramic obtained b y  melt-quenching. Fig. 4 shows the photomicrograph of the grown crystals. The single crystal X-ray diffraction studies indicate that the grown crystals are single crystalline (Fig.5) with unit cell parameters a = 16.573(4), b = 5.605(1) and c = 15.284(3) A and the large faces of the platelets are parallel to the crystallographic plane (001). The present data is consistent with that of Abraham et  all’. The capacitance and the dielectric loss (tans) measurements at 100 kHz have been ca,rried out on conventionally sintered c.eramic, partially grain-oriented ceramic and single crystals (along c-axis) as a function of pressure at room temperature. The dielectric constants have been evaluated by making correction for stray capacitance introduced by the leads etc. Fig. 6 shows the variation of the dielectric constants for the single crystal, conventionally prepared ceramic and partially grain-oriented ce- ramic as a function of pressure. The dielectric data has been recorded while reloading 90 Ramasesha et al the samples. The dielectric constants obtained for all these samples at atmospheric Figure 4: Photograph of as grown single crystals of Biz VO5.s Figure 5: Oscillatzon photograph of the grown crystals pressure are in agreement with those of the values obtained by extrapolating the curves (dotted curves) on to the Y-axis. The dielectric constant in all the three cases decreases with increasing pressure. However, the dispersion of the dielectric constant with pressure in the case of the partially grain-oriented ceramic is higher. The dielectric constant of the partially grain-oriented ceramic is higher up to the pressure of about 1 GPa than that of the conventionally prepared ceramic and sin- gle crystal. The dielectric loss (tan 6) also decreases with increasing pressure in all the cases (Fig.7). No sudden changes either in t, or tan 6 have been noticed in the range of pressures under study, indicating that the pressures employed in the Dielectric properties of Biz V05.5 under pressure 91 present studies are not adequate to induce any phase transitions. It is known that the dielectric constant can be written in terms of the individual polarizabilities of the ions. Since, the present dielectric constant measurements have been carried out at 100 kHz, the electronic polarizability contribution to the observed dielectric con- stant is less significant. Therefore, the only significant contribution could be from the ionic polarizability. With increasing pressure the ions in the lattice get cioser and as a consequence the elastic restoring forces increase, Hence, ionic polarizability Fig. 6 Fig. 7 POLYCRYSTALLINE O SINGLE CRYSTAL O SINGLE CRYSTAL v) 2 z 0 2 4 0 2 4 PRESSURE ( GPa) PRESSURE ( GPa ) Figure 6: The variation of dielectric constant with pressure for  the ceramics and the single crystals (c-axis). Figure 7:.  The variation of the dielectric loss (tan 6) with pressure for the ceramics and the single crystals (c-axis). which arises due to the ionic displacements and the resulting displacement of the electronic charge distributions can be expected to decrease. Consequently the dielec- tric constant decreases with increasing pressure. At macroscopic level the decrease in dielectric constant with increasing pressure may also be attributed to the decrease in an orthorhombic distortion (b/a ratio). 92 Ramasesha et al References 1. G.A. Samara, Advances in high pressurn research, ed. R.S. Bradley. vo1.3, 1969. 2. G.A. Samara, Phys. Rev., 151, 378, (1966). 3. B. Aurivillius, Arkiv Kemi, 1, 499, 463, (1949). 4. B. Aurivillius, Arkiv Kemi, 2, 519, (1950). 5. V.N. Borisov, Y.M. Poplavko, P.B. Avakyan and V.G. Osipyan, Sou. Phys.: Solid 6. A.A. Bush and Yu.N. Venevstev, Russ. J. Inorg. Chem., 31, 769, (1986). 7. V.G. Osipyan, L.M. Savchenko, V.L. Elbakyan and P.B. Avakyan, Inorganic Mate- 8. K.B.R. Varma, G.N. Subbanna, T.N. Guru Row and C.N.R. Rao, J. Mater. Res., 9. K.V.R. Prasad and K.B.R. Varma, Mat. Chem. and Phys., 38,406, (1994). State, 30, 904, (1988). rials, 23(3), 467, (1987). 5(11), 2718, (1990). 10. K.V.R. Prasad and K.B.R. Varma, J. Mater. Sci., (to be published). 11. F.K. Lotgering, J.  Inorg. Nucl. Chem., 9, 113, (1959) .  12. F. Abraham, M.F. Debreuille-Gresse, G. Mairesse and G. Nowogrocki, Solid State Ion., 28-30, 529, (1988). 

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The dielectric properties of ferroelectric bismuth vandate,Bi2 VO5.5 under pressure

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