Article pubs.acs.org/cm
Ferroelectric Response Induced in cis-Type Anion Ordered SrTaO2N Oxynitride Perovskite Shinichi Kikkawa,*,† Shikuan Sun,† Yuji Masubuchi,† Yuki Nagamine,‡ and Takeshi Shibahara‡ †
Faculty of Engineering, Hokkaido University, N13W8, Sapporo 060-8628, Japan TDK Corporation, Technical Center, 2-15-7 Higashi-Ohwada, Ichikawa, Chiba 272-8558, Japan
‡
S Supporting Information *
ABSTRACT: Oxynitride perovskite SrTaO2N has been attracting attention as a possible new dielectric material owing to the cis-type anion ordering in its crystal structure. It is currently difficult to obtain it in bulk form because it easily loses a part of its nitrogen to become semiconducting during densification at high temperature. We found that its surface layer recovered its original orange color and dielectric properties after postannealing in ammonia. Piezoresponse force microscopy measurements clearly revealed a ferroelectric behavior on the entire orange surface layer peeled off from the black sintered body inside. The present sample was freestanding at about 8 μm thick and clearly distinct from the compressively strained SrTaO2N thin films for which ferroelectricity was recently reported in small domains (10−102 nm) in the locally assumed trans-type anion ordering. The present results represent the first experimental observation of ferroelectric response in a bulk oxynitride perovskite with cis-type anion ordering.
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INTRODUCTION Piezoelectric ceramics are widely used, including in our daily lives, and their applications are expected to expand even further. Currently, the most widely used piezoelectric material is the oxide lead zirconate titanate (PZT) (piezoelectric constant d33 = 400−500 pm/V). However, it is a controversial material because it is hazardous to our health and to the environment. Oxynitride perovskites are attracting attention as a potential alternative. High bulk dielectric constants, κ ≈ 4900 and 2900, were reported respectively for BaTaO2N and SrTaO2N ceramics at room temperature.1 These values are comparable to or better than that of PZT and relatively weakly temperaturedependent, with no sign of a phase transition over the temperature range 180−300 K. The porosity was only about 45% even after heating the isostatically pressed compacts under flowing ammonia at 1293 K for 2 h. Anion ordering was found in SrMO2N (M = Nb, Ta) using neutron and electron diffraction.2 A slight tetragonal distortion was present at room temperature in the high-temperature phase in cubic Pm3m ̅ symmetry above 573 K for SrNbO2N and above 473 K for SrTaO2N. Refinements of the room-temperature tetragonal P4/mmm model revealed the possibility of a 1:2 anion order, on the basis of results indicating that the 1b site fully was occupied by O and the 2f site was occupied by a nearly 50/50 O/N mixture. The robust 1O/2(O0.5N0.5) partial anion order was assumed to be present up to at least 1023 K. The room-temperature superstructure in the I4/mcm model led to rotations of the MO4N2 octahedra with local cis-ordering of the two nitrides in each octahedron driven by covalency. The ordering resulted in disordered zigzag M−N chains within the © XXXX American Chemical Society
perovskite lattice, inducing off-centered displacements and local dipoles at each octahedron. Neutron diffraction data for SrTaO2N were almost simultaneously refined in the tetragonal I4/mcm system.3 The refined anion occupancies were approximately 50/50 in the 4a sites and 75/25 O/N in the 8h sites and strongly supported the cis-ordering of two nitride anions in an octahedron around Ta5+. Both the local polarity and the tilting of the octahedron are thought to induce dielectric properties in SrTaO2N. A first-principles study showed that stable anion orderings in BaTaO2N and SrTaO2N have two kinds of similar three-dimensional −Ta−N− coiled chain motifs that can transform into each other, providing a mechanism to break long-range ordering and increase the diversity of anion ordering around the pentavalent tantalum.4 Both materials have two sets of low-energy displacements forming opposite polarization directions that can be easily altered at the picosecond scale. This was assumed to be the cause of relaxor-type behavior without chemical inhomogeneity. To increase the dielectric constant of a ceramic material, its porosity needs to be minimized. The above-mentioned porosity value of about 45% means that the materials were not sintered well enough to obtain necking between grains.1 Densification, while it preserved the chemical composition observed before sintering, was not easily achieved in oxynitrides, although their crystal structure was confirmed to have perovskite structure. They readily release a certain amount of nitrogen from their Received: October 27, 2015 Revised: February 16, 2016
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DOI: 10.1021/acs.chemmater.5b04149 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials perovskite crystal lattice and are reduced.5,6 This partial loss of nitrogen was recently studied in detail in the case of both SrTaO2N and LaTiO2N.7 SrTaO2N heated in a helium atmosphere lost about 30 atom % of its nitrogen at 1223 K, while maintaining its perovskite structure and changing its color from orange to dark green. Then, above 1473 K, it decomposed into a black mixture of Sr1.4Ta0.6O2.73, Ta2N, and Sr5Ta4O15. This second decomposition was not clearly observed, especially when SrTaO2N was heated in a nitrogen atmosphere below 1823 K. After heating at 1773 K for 3 h under a 0.2 MPa nitrogen atmosphere, the perovskite product became dark green and electrically conductive. Its structure refinement resulted in a mixture of tetragonal and cubic perovskites with a decreased ordering of N3−/O2−. Disordered cis-anion chains have been revealed on SrTaO2N and LaTaON2 in hightemperature (up to 1373 K) neutron diffraction.8 These ceramics were reduced from their initial insulating state to an electrically conductive state during sintering. Pressureless sintering of SrTaO2N has been investigated.9,10 Owing to the low thermal stability of SrTaO2N, the partial loss of SrO and nitrogen induced the formation of a TaO0.9 impurity after heat-treating above 1373 K. Single-phase perovskite oxynitride SrTaO2N ceramics were prepared by sintering at 1673 K under a nitrogen atmosphere, using 2.5 wt % SrCO3 as a sintering additive. The sintering additive and postannealing in NH3 helped to compensate for the loss of SrO and nitrogen and yielded ceramics with the original chemical composition, although a similar partial loss was observed even in the case of the glass-encapsulated HIPing technique.11 Assintered SrTaO2N ceramics with various relative densities (RDs) were annealed in NH3 to observe the recovery of color and electrical insulativity. The original orange color was recovered on the surface, but not the interior, of well-sintered samples with RD = 95.1%. The interior continued to exhibit semiconducting behavior and a black color. On the other hand, for as-sintered SrTaO2N ceramics with RD < 84%, both the nitrogen content and electrical insulation behavior were completely recovered after annealing. Postannealed SrTaO2N ceramics (RD = 83.3%) possessed a relatively large dielectric constant of 450 with a low dielectric loss of less than 0.1 at 100 Hz, almost independently of frequency and temperature. Thin films of the oxynitride perovskites have also been studied as alternatives for sintered bulk materials because it is difficult to ensure that they maintain their initial composition even when sintered.12,13 The possibility of ferroelectricity was reported on compressively strained SrTaO2N thin films epitaxially grown on SrTiO3 (STO) substrates using nitrogen plasma-assisted pulsed laser deposition.14 Piezoresponse force microscopy measurements revealed small ferroelectric domains (10−102 nm). The ferroelectricity observed on the strained films was inhomogeneous. The surrounding matrix region exhibited relaxor ferroelectric-like behavior, with remnant polarization invoked by domain poling. First-principles calculations performed among hypothetical crystal structures (Pbmm cis, I4/mcm trans, P4mm trans) suggested that the small domains and the surrounding matrix had a trans- and cis-type anion arrangement, respectively. The structural assumption contradicts the crystal structure of SrTaO2N (I4/mcm) already confirmed.2,3 In the present paper, we report room-temperature ferroelectricity in the free-standing SrTaO2N thick layer in cis-type anion ordering. Piezoresponse force microscopy (PRM) measurements clearly reveal ferroelectric behavior on
the entire orange surface layer peeled off from the inner black sintered ceramics after postannealing in flowing ammonia. The present results represent the first report for ferroelectricity in oxynitride perovskite bulk ceramics.
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EXPERIMENTAL SECTION
Sample Preparation. A stoichiometric mixture of SrCO3 and Ta2O5 (both are 99.9%; Wako Pure Chemicals Co. Ltd., Osaka, Japan) was calcined at 1473 K for 12 h in air to prepare the Sr2Ta2O7 precursor. This precursor was nitrided at 1273 K in a 100 mL/min ammonia flow for 80 h using a rotating tube furnace (MS-2576; Motoyama Co. Ltd., Osaka, Japan). Intermittent grinding was performed every 20 h. Then, 2.5 wt % SrCO3 was mixed with the as-prepared SrTaO2N powder as a sintering additive. The mixture was uniaxially pressed at 10 MPa, cold isostatically pressed at 150 MPa, and sintered in a graphite furnace (High Multi 500; Fuji Dempa Kogyo Co. Ltd., Osaka, Japan). The compacts were placed in a boron nitride crucible and sintered at 1723 K for 3 h under 0.2 MPa of nitrogen pressure. The heating rate was maintained at 20 K/min below 1473 K and at 10 K/min above 1473 K. After being kept at 1723 K for 3 h, the furnace was cooled naturally to room temperature. Polishing was performed with a 400-grit sandpaper (particle size: 3 μm) to obtain flat parallel surfaces on the two sides of the sintered body to enhance the accuracy of the X-ray diffraction. After polishing, the assintered ceramic was annealed at 1223 K for 20 h in NH3 flow (100 mL/min) to recover the stoichiometric nitrogen amount. The relative density of the as-prepared ceramic was determined using a water vacuum penetration technique in combination with Archimedes method. The as-received powder and the polished surface of the ceramic were characterized by XRD with Cu Kα radiation at 40 kV and 40 mA (Ultima IV; Rigaku Co. Ltd., Tokyo, Japan). Atomic Force Microscopy (AFM) and PRM. The specimen was fixed on a 10 × 2 × 1.5 mm conductive ceramic block by means of a conductive epoxy paste (Circuit Works, Chemitronics). Then, the specimen was tilted slightly (by less than 1°) in advance and polished by a lapping sheet (Lapping Film, 3M) in order to control the specimen thickness to obtain a sufficient applied electrical field for observing the piezoresponse. The conductive ceramic block with the lapped specimen was stabilized on a 10 × 10 mm copper plate for AFM observation by means of a conductive silver paste (DOTITE, Fujikura). Atomic force microscopy (AFM) and PRM measurements were conducted using a scanning probe microscope (E-sweep with NanoNavi Probe Station, SII Nano Technology). A Rh-coated silicon cantilever (Si-DF3-R(100), SII Nano Technology) with a stiffness of 1.7 N/m and a resonance frequency of 27 kHz was employed for the measurements. In the PRM measurements, the frequency of the driving AC electrical field was set to 5 kHz, and the amplitude of the applied AC field was 1−10 V. The domain polling process was performed by applying biases of −10 and 10 V to each measurement area. Local Dielectric Property Measurements. Pt deposition was made to form electrode pads for dielectric property evaluation. The pad sizes were 15 × 15 μm with 0.6 μm thick for hysteresis measurement and 20 × 20 μm with 1.5 μm thick for dielectric measurement, respectively. Two Pt pads with different thicknesses were formed on the specimen for the dielectric measurement. These were processed by Focused Ion Beam (FIB), (Nova200i NanoLab, FEI Company, USA). In order to avoid surface leakage caused by Pt diffusion from the pads, the surroundings of the pad were grooved. In the SEM measurements, the specimen thicknesses were 10 μm for the hysteresis and 8.9 and 6.3 μm for the P−E measurements, respectively. The dielectric constant and loss tangent were measured using an Agilent Technologies Impedance Analyzer 4294A at 0.1−1000 kHz and 500 mV ac amplitude. Hysteresis measurements were carried out at 400 Hz using a ferroelectric test system (FCE-1; TOYO Corporation, Tokyo, Japan) at room temperature. B
DOI: 10.1021/acs.chemmater.5b04149 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
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RESULTS SrTaO2N with 94.5% relative density was obtained by firing a cold isostatically pressed powder compact under 0.2 MPa N2 atmosphere at 1723 K for 3 h.10 The color of the sample changed from orange to black, as shown in Figure 1, and its size
Figure 3. PRM sample preparation from the postannealed SrTaO2N sample (a) and optical micrograph of the polished sample (b). The orange area inside the red square was observed by VPRM.
Figure 1. SrTaO2N samples: Green compact (a), as-sintered ceramic (b), and ceramic annealed in ammonia (c).
the volume change due to the ammonia annealing treatment. The tetragonal lattice parameters were a = 0.57001(2) nm, c = 0.80715(4) nm, V = 0.26229(2) nm3 before, and a = 0.57017(1) nm, c = 0.80789(2) nm, V = 0.26264(1) nm3 after annealing in ammonia. Cracking occurred at the interface due to the stress between the orange annealed surface and the black ceramic inside, induced by the slight volume change. The small piece with an estimated thickness of 8 μm was used to confirm the reproducibility of the above VPRM experimental result. The measurement in the vertical direction was performed on the peeled off surface sample at the edge area marked by a circle in Figure 5 after poling in a stripe pattern under a ± 10 V DC voltage. The polarized pattern was clearly observed at measured voltages of both ±3 V and ±4 V as depicted in Figure 6, similarly to the above-mentioned observation of the inclined slice. The piezoresponse image gradually became less clear with a further increase in the measured voltage. Then, it almost disappeared above ±7 V, suggesting that the coercive electric field is around these voltages. The slight surface roughness in the topographic image can be seen together with the vertical piezoresponse at ±8 V. A local hysteresis loop was observed within ±12 V using Ptdeposited electrode (15 × 15 × 0.6 μm) as shown in Figure 7 similarly to the observation in thin film.14 Current leakage began at the coercive electric field and became serious above ±12 V. The local dielectric property was measured on the similar sample arrangement with different thicknesses of 8.9 and 6.2 μm against frequency as represented in Figure 8. There was no particular change in dielectric constant ε above 100 Hz where charge motion generally responds. Values for both dielectric constant ε and dielectric loss tan δ were almost independent against frequency in the range of ∼104−106 Hz. The dielectric constant ε was similar or much smaller and the dielectric loss tan δ was much larger than those reported in the previous report on thin film.14 Ferroelectric characteristics mainly contribute to the large ε value in the SrTaO2N surface layer even though there might be present some amount of charge motion because of the relatively large porosity.
shrank slightly, from 6 to 5 mm in diameter and from 1.2 to 1.0 mm in thickness. The sample regained its initial orange color and tetragonal lattice parameters upon subsequent annealing in flowing NH3 at 1223 K for 12 h. The interior of the annealed ceramic was still black, and the orange surface layer was about 8 μm in thickness as seen in Figure 2. The relative density needed to be less than 85% for complete color recovery, including the interior of the sample.7,10 The annealed ceramic surface was cut at a slight incline to increase the area for contact-resonance-mode vertical piezoresponse force microscopy (hereafter VPRM), as shown in Figure 3a. The slice obtained was polished after being attached to a sample holder with conductive paste. A small area of the orange edge of the polished thin section in Figure 3b was observed under vacuum (
