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Ferroelectricity: Measurement of the Dielectric Susceptibility of Strontium

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Ferroelectricity: Measurement of the Dielectric Susceptibility of Strontium Ferroelectricity: Measurement of the dielectric susceptibility of strontium titanate at low temperatures Matthew Trainera) Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom ͑Received 30 August 2000; accepted 28 February 2001͒ A cryogenic experiment determining the dielectric susceptibility of the ferroelectric substance, strontium titanate, in the 90- to 300-K temperature range is described. Evidence is presented for a structural phase transition in strontium titanate at 110 K. The experiment is suitable for advanced students. © 2001 American Association of Physics Teachers. ͓DOI: 10.1119/1.1374251͔ I. INTERESTING AND UNUSUAL PROPERTIES OF temperatures. Its properties at very low temperature also in- FERROELECTRICS clude superconductivity9 and quantum paraelectric behavior.10 Ferroelectricity describes a collection of phenomena Observations of the properties of SrTiO3 have played an which contribute to the generation of a spontaneous electric important role in the development of the theory of structural polarization in materials called ferroelectrics.1 Spontaneous phase transitions. It was the first material for which a high static dielectric constant at low temperature was associated polarization is a result of the materials having a unique polar ͑ ͒ 11 axis. The important point is that the direction of the sponta- with a low frequency ‘‘soft’’ optical mode. This gave experimental proof of the Lyddane–Sachs–Teller relation.12 neous polarization can be switched by an external electric Indeed, soft modes were first clearly measured in a lattice field. dynamics study of the SrTiO phase transition.13 SrTiO was Ferroelectrics are an extremely important class of materi- 3 3 the first crystal in which a central peak was revealed by als used in the electronics industry. The main interest is in neutron scattering.14 Also x-ray scattering studies of the the polycrystalline ͑ceramic͒ and thin film forms of the ma- SrTiO phase transition brought into existence the concept of terials. These are easier to make than single crystals and their 3 a two-length scale behavior arising from defects and dislo- composition can be readily modified. However the physical cations in the crystal structure.15 properties of the ceramic form are different from those of the The aim of this experiment is to measure the dielectric single crystal and depend on parameters such as doping, 16 2 susceptibility of SrTiO3 at low temperature and look for grain size, and structural defects. Ferroelectrics are used as evidence of the structural phase change. dielectrics in ceramic capacitors and in piezoelectric devices. Exciting new possible uses for ferroelectric thin films in- II. DETERMINATION OF THE DIELECTRIC clude nonvolatile memory applications and microelectro- SUSCEPTIBILITY mechanical systems.3 Pray4 described a method of measuring the dielectric con- An easy way of determining the dielectric susceptibility is stant of thin films of the dielectric cryolite (Na3AlF6) using a to measure the capacitance of a parallel plate capacitor con- 17 parallel plate capacitor technique and a balanced capacitance taining the ferroelectric substance as a dielectric. The bridge. Morgan et al.5 measured the pyroelectric coefficient sample is a 100-oriented slice (10 mmϫ10 mmϫ0.27 mm) 18 and spontaneous polarization of the ferroelectric triglycine of a single crystal of pure strontium titanate. Gold elec- sulfate ͑TGS͒ from 25 to 30 °C and observed a phase change trodes of 6.8 mm diameter are vacuum evaporated on both at 49 °C. Pyroelectric materials are useful as infrared light sides of the sample to form the capacitor. If vacuum evapo- ␮ detectors because they possess the characteristic of a ration equipment is not available then 50- m copper foil can temperature-dependent spontaneous polarization. All ferro- be attached with a thin layer of Opticlean fluid®. Silver paste electric materials are pyroelectric. Ofer and Jodl6 investi- is used to attach fine enameled copper wire to the electrodes. gated the electro-optic effects of the ceramic ferroelectric For the parallel plate capacitor, the polarization P induced in the SrTiO dielectric is proportional to the applied electric known as PLZT ͓a modified form of Pb͑Zr, Ti͒O ͔. The 3 3 field vector E, transparency of this substance varies under the influence of ϭ␹ ⑀ ͑ ͒ an electric field. The effect is temperature dependent. Appli- P e 0E, 1 cations for this material include welding goggles and inten- ⑀ ϭ where the dielectric permittivity of a vacuum 0 8.85 sity modulation. Ϫ Ϫ ϫ10 12 Fm 1. Relation ͑1͒ is valid only for linear material A ferroelectric of particular interest because of its unusual and the dielectric susceptibility ␹ is a dimensionless quan- properties at low temperatures is strontium titanate, SrTiO . e 3 tity representing an isotropic medium with a cubic structure. SrTiO3 belongs to a family of compounds with the general Note, however, that for an anisotropic medium the dielectric formula ABO , where A is a divalent ion and B is a ␹ Ϫ1 3 susceptibility is a second-rank tensor ij (Fm ). Perkalskis 1 19 quadrivalent ion. At room temperature SrTiO3 has a cubic and Freeman demonstrated the tensor properties of aniso- perovskite-type structure but at ϳ105 K undergoes a phase tropic dielectric materials to great effect using a cylindrical transition to a tetragonal form.7 Grupp and Goldman8 ob- capacitor. The dielectric displacement vector D represents served a giant piezoelectric effect in SrTiO3 at cryogenic the total surface charge density induced in the dielectric 966 Am. J. Phys. 69 ͑9͒, September 2001 http://ojps.aip.org/ajp/ © 2001 American Association of Physics Teachers 966 Fig. 2. Dielectric susceptibility of SrTiO3 vs temperature. Fig. 1. Setup for determining dielectric susceptibility of SrTiO3. and test fixture can be determined. This was ϳ80 pF in the 90- to 300-K range. The stray capacitance is subtracted from the measured capacitance to give the corrected value. ϭ⑀ ϩ ͑ ͒ D 0E P. 2 The dielectric susceptibility is complex with a real ␹Ј and Substituting the term for P in Eq. ͑2͒ then imaginary component ␹Љ: ϭ⑀ ϩ␹ ⑀ ϭ ϩ␹ ͒⑀ ͑ ͒ D 0E e 0E ͑1 e 0E. 3 ␹ϭ␹ЈϪi␹Љ. ͑6͒ If Q is the total charge on the parallel plate capacitor and A is Equation ͑5͒ is used to convert the measured capacitance the area of the gold electrode, then DϭQ/A. Also EϭV/d value to the real dielectric susceptibility. The imaginary di- where V is the potential difference across the capacitor and d electric susceptibility is calculated from the dissipation fac- ␦ϭ␹Љ ␹Ј ␦ is the thickness of the SrTiO3 sample. Substituting these re- tor, tan / . tan is a measure of the energy lost per lationships for D and E in term ͑3͒, the following equation is cycle in the form of heat. obtained: A plot of real and imaginary dielectric susceptibility ver- sus temperature at 1 kHz and zero dc bias field is shown in Q V ϭ͑ ϩ␹ ͒⑀ ͑ ͒ Fig. 2. The ␹Ј values compare well with those measured in 1 e 0 . 4 22 A d other studies. Dielectric spectroscopy studies of SrTiO3 showed that ␹Ј is almost frequency independent in the range Now the capacitance C of the parallel plate capacitor is de- Ϫ fined as the ratio of the charge on either plate to the potential 10 3 –108 Hz at all temperatures.23 Dec et al.24 clearly dem- difference between the plates, CϭQ/V. From this, the rela- onstrated the scaling behavior of ␹Ј with respect to dc bias tionship between capacitance C and dielectric susceptibility field at temperatures of Ͻ65 K. This ability to modify the ␹ e is expressed as dielectric susceptibility of ferroelectric materials by an ap- plied electric field has created some excitement in micro- Q 25 17 Cϭ ϭC ͑1ϩ␹ ͒, ͑5͒ wave engineering. Neville et al. observed that between V 0 e 65 and 300 K, ␹Ј is independent of applied dc field. In con- 26 ϭ⑀ 20 trast, van der Berg et al. observed field-dependent permit- where C0 0A/d and is the capacitance with no dielectric. ϳ tivity for Nb-doped SrTiO3 crystals at room temperature. The C0 was calculated as 1 pF. ␹ ␹ losses Љ are very low for SrTiO3, rising from 0.5% Ј at Figure 1 shows the measurement setup. The sample is ␹Ј ϳ ␹Љ mounted on a copper block inside the vacuum can of a cry- 300 K to 0.7% at 90 K. This rise in near the struc- tural phase transition is due to dielectric relaxation and is ostat. By evacuating the can, all air moisture is removed. 27 Moisture solidifying on the sample when the temperature is frequency dependent. Low loss is a very desirable feature lowered would interfere with its electrical properties. A in a dielectric. High loss causes heating of the capacitor T-type thermocouple attached to a corner of the sample and which may lead to a destructive breakdown of the dielectric connected to a digital thermometer measures the sample tem- under certain circumstances. perature. Capacitance and dissipation factor tan ␦ measure- ments are taken using a digital LCR meter with an ac test signal of 0.9-V rms amplitude and 1-kHz frequency. A test III. THE PHASE TRANSITION IN STRONTIUM fixture interfaces the sample leads to the LCR meter.21 TITANATE After evacuating the vacuum can, the copper rod of the cryostat is immersed in liquid nitrogen to bring the tempera- The dielectric susceptibility values for SrTiO3 can be used ture of the sample down to ϳ90 K. The cryostat is then to derive useful information on its low temperature phase removed from the liquid nitrogen and allowed to warm up. transition. SrTiO3, like other ferroelectrics, loses its intrinsic Measurements of C and tan ␦ are taken at frequent intervals polarization above a transition temperature and becomes from 90 to 300 K.
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