Physical and electrical properties of SrTiO3 and SrZrO3

Perovskite type oxide strontium titanate (SrTiO3) and strontium zirconate (SrZrO3) ceramic powder has been synthesized using conventional solid state reaction method. The powders were mixed and ground undergone calcinations at 1400oC for 12 h and sintered at 1550oC for 5h. X-ray Diffraction exposes physical properties SrTiO3 which exhibit cubic phase (space group: pm-3m) at room temperature meanwhile SrZrO3 has Orthorhombic phase (space group: pnma). The electrical properties such as dielectric constant (εr), dielectric loss (tan δ), and conductivity (σ) were studied in variation temperature and frequency. High dielectric constant of SrTiO3 and SrZrO3 were observed at 10 kHz for both samples about 240 and 21 respectively at room temperature. The dielectric loss of SrTiO3 and SrZrO3 is very low loss value approximately 0.00076 and 0.67512 indicates very good dielectric.


Introduction
Ceramic with perovskite crystal structure have long been investigated due to their excellent ferroelectric, paraelectric properties and chemically stable-structure. Its applicable in displays, Multilayer Ceramic Capacitor (MLCC), electronic/piezoelectric devices, sensors, actuators, transducers, wireless communications [1][2][3][4][5]. The ideal Perovskite structure, which have the general formula ABX3 are consisting of two different cations (A and B) in equal ratio and an anion (X), which is usually oxygen.
SrTiO3 and SrZrO3 are such materials that have been current interest to study due high dielectric constant and its a lead free material. At room temperature, SrTiO3 has a simple cubic perovskite structure with space group (pm-3m) meanwhile SrZrO3 has orthorhombic structure with space group (pnma) but both composition undergoes a series of phase transitions by varying the temperature. SrTiO3 undergoes phase transitions from high to low symmetry [6], while low to high symmetry for SrZrO3 [7].
SrTiO3 and SrZrO3 is a perovskite dielectric material applied to many application fields such as integrated microelectronic and microwave device [8]. These features are attributed by their unique properties such as high dielectric constant, low dielectric loss, tunability, high breakdown strength and low leakage current density [9][10][11]. Besides, the melting point of SrTiO3 and SrZrO3 is 2080 o C and 2600°C respectively, making it applicable at high temperatures. In 2001, Shende et al. [11] claim that SrTiO3 and SrZrO3 can be used in high-voltage applications because of its high breakdown strengths which these materials are not expected to experience electromechanical failure mechanisms that may result in dielectric breakdown. Parida et al. [12] in 2012 report that SrTiO3 and SrZrO3 has high permittivity and low loss make it suitable for microwave antenna application. Based on Wang et al [13] study, high dielectric constant and favourable bias stability inside SrTiO3 are widely required for the application of high energy storage density dielectrics.
Numerous attempts have been explored to further improve the properties of SrTiO3 based ceramics. Among them, doping was considered as an effective approach for altering their properties. ZrO2 was selected as cation substitute the B-type site of SrTiO3, because Zr 4+ ions in SrTiO3 can stabilize the charge of Ti 4+ and suppress the oxygen dissociation when sintered at high temperature [13]. SrTiO3-SrZrO3 solid solution is one of them, which shows a super lattice structure due to its cell enlargement as a result of tilting of BO6 (B = Ti, Zr) octahedral [14]. This type of disorder perovskite solid solution offers exciting new possibilities both in the investigation of fundamental physical behaviour and in the exploitation of novel properties for various applications.
In this study, the electrical properties of SrTiO3 and SrZrO3 prepared via solid state reaction method were investigated as respect to its physical structure properties. So, the system is expected to provide access to the desired phase or properties at convenient temperatures.

Experimental procedure
The Perovskite-type oxide were prepared by mixing high purity SrCO3 (99.9% Aldrich), TiO2 (99.0% Sigma Aldrich), and ZrO2 (99% Aldrich) in the appropriate stoichiometric ratios. Sample was mixed and ground using pestle and mortar with acetone as mixing medium. The obtained powders were pressed into pellets of diameter 13mm and heated at high temperature 1400 o C for 12h with heating rate 5 o C/m in the furnace. The samples were sintered for 1550 o C for 5h in air followed by characterization.
X-ray Diffraction (XRD) were conducted at room temperature to determine the crystalline structure of all samples by using X-Ray Diffractometer (Model: D2 PHASER from Bruker AXS) in a 2θ range from 10 o to 80 o , using Cu Kα1 radiation. The electrical properties such as dielectric constant (εr), dielectric loss (tan δ), conductivity(σ) as well as impedance (Z) were studied by making use of impedance-capacitance-resistance (LCR) meter (Model: HIOKI 3522-50 LCR HiTESTER) at temperature 30-600 o C. For this purpose, sintered pallets were coated with silver paste as electrode. Samples are measured over the frequency range of 10 Hz -100 kHz with different temperature.

Results and Discussion
The structures of the oxide SrTiO3 and SrZrO3 were first studied using powder X-ray diffraction data within 10° to 80° value of 2θ in range as shown in Figure 1(a) and   1(b). All the peaks are sharp and there is no unwanted peak is found in XRD pattern, representing the crystal is single phase without any impurities and identical to the pdf reported [SrTiO3=PDF00-035-0734] and [SrZrO3= PDF01-076-9442]. From the diffraction pattern, the lattice parameter along with the average crystallite size for both the samples has been calculated as shown in Table 1. SrTiO3 was indexed Cubic phase (space group: P m -3 m) with unit cell with lattice constant formulated by a = 3.9000Å which are in contrast to the orthorhombic structure of the SrZrO3 with unit cell with lattice constant formulated by a = 5.7915Å, b=5.8129Å, c=8.2000Å.
The Dielectric Constant (εr) was calculated from the measured capacitance (Cp) obtain by LCR meter between 30 -600 o C temperatures. The variations of dielectric constant with temperature at some selected frequencies for SrTiO3 and SrZrO3 ceramics are shown in Figure 2(a) and 2(b). For SrTiO3 ceramics, the dielectric constant decreases gradually up to a certain temperature and increases rapidly with increasing temperature. Similar phenomenon of the dielectric constant was observed in SrZrO3 ceramics. There is no curie peak detected for both sample in the whole measurement temperature range. The dielectric constant of SrTiO3 much higher compared to SrZrO3 .dielectric constant is decrease in from ɛr = 240 (SrTiO3) to ɛr = 21 (SrZrO3) is due to the decrease in ionic polarization [15].

Figure 3(a) and 3(b)
shows the Capacitance against frequency, the spectrum displace toward higher frequency. SrTiO3 exhibit capacitance of average 10 -9 ~ 10 -10 F, meanwhile SrZrO3 show to have capacitance value in range of 10 -10 ~ 10 -11 F, Thus the range of capacitance probably belonged to grain boundary [16]. The highest capacitance value is obtain at temperature 600 o C which give 2.9x10 -9 F for SrTiO3 and 1.3x10 -10 F for SrZrO3 at 1 kHz. The capacitance related to the ability of SrTiO3 and SrZrO3 sample to collect and store energy in the form of an electrical charge.  EPJ Web of Conferences 162, 01052 (2017) DOI: 10.1051/epjconf/201716201052 InCAPE2017 value fluctuate and increased at about 1. However, for SrZrO3 below 300 o C the dielectric loss is less than 1 but increasing after 300 o C. The dielectric loss of SrZrO3 ceramics seems to remain similar trend but it gives much higher of loss as compared to SrTiO3.   The frequency dependent conductivity for SrTiO3 and SrZrO3 in the temperature range of 300-600°C was shown in Figure 5(a) and 5(b). The conductivity observed for SrZrO3 slightly lower as compared to pure SrTiO3. For SrTiO3 it shows that the conductivity increased by increasing temperature which behave like typical dielectric ceramic. The dc conductivity was about 4.9x10 -9 Scm -1 to 7.7x10 -6 Scm -1 and 5.3x10 -9 Scm -1 to 5.2x10 -6 Scm -1 for SrZrO3. SrZrO3 show the conductivity behaviour at frequency 10 Hz and 10 kHz. This phenomenon are related to the conductivity of grain boundaries as reported by Liu [17]

Conclusions
SrTiO3 and SrZrO3 was phase pure after heating at 1400 o C for 12h. SrTiO3 exhibit cubic structure with space group pm-3m however SrZrO3 exhibit orthorhombic structure with space group pnma. This is proven by the change in unit cell and volume of the samples. In term of the dielectric properties, SrTiO3 have much higher value of dielectric constant which is 240 compare to SrZrO3 is 21 at 10 kHz. The dielectric loss of SrTiO3 is 0.00191 and SrZrO3 is 0.26547. Besides, at 400 o C the conductivity of SrTiO3 is 7.8x10 -6 Scm -1 and 3.2x10 -6 Scm -1 for SrTiO3. Overall the properties of SrTiO3 is much better than SrZrO3.