Communication pubs.acs.org/cm
High-Dielectric-Permittivity Layered Nitride CaTiN2 Xiaohui Li,†,‡ Xiaoming Wang,*,‡ Yifeng Han,† Xiping Jing,‡ Qingzhen Huang,§ Xiaojun Kuang,*,† Qilong Gao,∥ Jun Chen,∥ and Xianran Xing∥ †
Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Nonferrous Metal and Featured Materials, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences, The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China § NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ∥ Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China S Supporting Information *
H
KCoO2-type layered nitride analogue to SrTiN2 but with a distorted orthorhombic structure. Black CaTiN2 compound was prepared by the hightemperature solid-state reaction at 1573 K using starting materials Ca3N2 and TiN in tungsten crucibles under reductive atmosphere of mixed 6% H2−94% N2 gas. The product were washed by using methanol solution of NH4Cl to remove the impurity CaO and dried in the vacuum oven. The powder X-ray diffraction (PXRD) pattern of CaTiN2 may be indexed with an orthorhombic unit cell of a ∼ 3.937 Å, b ∼ 3.613 Å, c ∼ 7.542 Å. The selected area electron diffraction (SAED) patterns (Figure 1) confirm this orthorhombic cell. Both the PXRD and
igh-dielectric-permittivity (εr) materials are key elements for miniaturization of microelectronics based on the capacitive components.1 The dielectric behaviors of the traditional oxide materials have been extensively investigated and the origins of high εr (>1000) have been classified into two catalogues in oxides.2 The first one is the intrinsic spontaneous ionic polarization in ferroelectric materials owing to their polar symmetries such as the displacive PbTiO33 and BaTiO34 based materials. The other one is the extrinsic polarization from the boundary regions between grains or inside grains as well as the sample−electrode interface. CaCu3Ti4O12 (CCTO) ceramic is a typical high-εr example on this extrinsic class owing to the internal barrier layer capacitance (IBLC) effect arising from the semiconducting grains and insulating grain boundaries.5,6 Nitrogen has higher effective ionic charge, larger ionic size, higher polarizability and lower electronegativity than oxygen, which makes (oxy)nitrides unique on crystal chemistry and physical properties that complement the traditional oxides in a variety of applications.7−10 In the past decades, interest in the metal (oxy)nitrides has been growing significantly owing to their inspiring hardness, luminescent, superconducting, thermoelectric, magnetoresistance and photocatalytic properties.8,9,11 However, the investigation on dielectric properties of the metal (oxy)nitrides is relatively little although the higher polarizability of N3− than O2− could make the (oxy)nitrides huge possibility for exploiting the high-bulk-εr materials. To our best knowledge, only a few perovskites,12−14 e.g., SrTaO2N and BaTaO2N have been reported as high-bulk-εr oxynitride ceramics.12,13 In this study, we demonstrate that remarkably high bulk εr ∼ 1300−2500 in 104−106 Hz over a wide temperature range of 80−450 K was achieved on a new two-dimensional (2D) layered nitride CaTiN2 and correlate the high bulk εr with the structural chemistry of CaTiN2. Ternary nitrides with AMN2 stoichiometry have been showed to adopt layered structures with several variants including α-NaFeO2,15 KCoO2,16 α-CuFeO217 and β-RbScO2 types.18 Among these layered structures, the KCoO2-type layered nitrides are quite few: only three tetragonal compounds SrTiN2,19 BaZrN220 and BaHfN221 in centric P4/nmm have been reported so far, which were shown to be potential thermoelectric materials with band insulator behavior from the theoretical calculation.22 Herein CaTiN2 represents a new © 2017 American Chemical Society
Figure 1. SAED patterns recorded along (a) [011̅] and (b) [111] for CaTiN2. The weak reflections marked by the arrows in panel a are due to the double diffraction.
the SAED patterns show the reflection conditions h00: h = 2n, 0k0: k = 2n and hk0: h + k = 2n, which suggest Pmmn and its subgroup P21mn or Pm21n as the possible space groups for CaTiN2. X-ray energy-dispersive spectroscopy (EDS) analysis of CaTiN2 (Figure S1) gave an average composition of CaTi0.96(6)N1.9(2), close to the nominal formula. The N content in CaTiN2 was also confirmed by oxygen-oxidation thermogravimetric analysis (TGA) and elemental analysis based on the combustion method. The TGA data in air of CaTiN2 sample Received: December 10, 2016 Revised: February 27, 2017 Published: February 28, 2017 1989
DOI: 10.1021/acs.chemmater.6b05226 Chem. Mater. 2017, 29, 1989−1993
Communication
Chemistry of Materials
Table 1. Final Refined Structural Parameters of CaTiN2a
showed a mass gain ∼17.2 wt % within ∼500−600 °C (Figure S2), which is ascribed to oxidation of CaTiN2 to CaO·TiO2 and consistent with the calculated value of 17.24 wt % based on the CaTiN2 composition. The elemental analysis gave the N content of 22.12(6) wt %, close to the calculated value (24.12 wt %) from CaTiN2 composition. Comparison of cell parameters between the orthorhombic CaTiN2 and the tetragonal SrTiN2 (a ∼ 3.8799 Å, c ∼ 7.6985 Å in P4/nmm)19 suggests that both CaTiN2 and SrTiN2 could have KCoO2-type layered structures except for the low symmetry for the Ca-phase. This was further evidenced by the plate-like morphology of CaTiN2 (Figure S3). Rietveld refinement of CaTiN2 was performed based on the PXRD data using an orthorhombic structural model that was derived from the tetragonal structure of SrTiN2. The highest symmetry Pmmn among the three possible space groups described above was employed for CaTiN2 (given that the second harmonic generation (SHG) measurement did not detect SHG effect on the CaTiN2 sample). The refinement converged to Rwp = 3.42% and Rp = 1.92% (Figure S4) and suggests significant preferred orientation (PO) along (00l) planes, owing to the layered structure nature. Neutron powder diffraction (NPD) data of CaTiN2 was then collected. No apparent PO effect was observed in NPD data therefore the final Rietveld analysis was performed on the NPD data only, which converged to Rwp = 2.66% and Rp = 2.12% (Figure 2), further confirming the
atom
site
x
y
z
Biso (Å2)
BVS
Ca Ti N1 N2
2b 2b 2a 2b
0.25 0.25 0.25 0.25
0.75 0.75 0.25 0.75
0.849(1) 0.414(2) 0.5255(7) 0.1632(8)
1.00(2) 0.72(8) 1.06(5) 1.02(8)
1.72 4.34 3.43 2.60
a
Space group: Pmmn, a = 3.9365(5) Å, b = 3.6131(5) Å, c = 7.5421(9) Å, Z = 2. BVS denotes bond valence sum.
Figure 3. (a) Structure of CaTiN2 viewed along [010] and the coordination environments of Ti and Ca to N in (b) Pmmn and (c) P21mn structures of CaTiN2. The N1 atoms marked by * have equal distances with Ti or Ca. The displacement directions marked by red arrows for Ca and Ti in panel c are parallel to the a-axis.
Such compressed apical distance along the c-axis compared with the basal metal−nitrogen bonds in the pyramid is similar to those observed in SrTiN219 and BaZrN2,20 but contrary to that in the essentially undistorted CoO5 pyramids in KCoO2.16 The coordination environment of calcium to nitrogen in CaTiN2 is also affected by the low symmetry, e.g., the centric structure shows 4 different Ca−N distances in the 9-coordinated Ca polyhedron (Figure 3b): one short apical Ca−N2 (∼2.36 Å), four long basal Ca−N1 bonds (∼3.03−3.44 Å) and four intermediate equatorial Ca−N2 bonds (∼2.67 Å). In the centric CaTiN2, all of basal Ti−N1, Ca−N1 and equatorial Ca−N2 bonds are symmetric around the apical M−N2 (M = Ti, Ca) bonds. Whereas the Ti and Ca atoms in the polar CaTiN2 are displaced along the a-axis by ∼0.025 and ∼0.13 Å, respectively. This leads to asymmetric basal Ti−N1 and equatorial Ca−N2 bonds around the apical Ti−N2 and Ca− N2 bonds (Figure 3c), respectively. Similar to SrTiN2,19 Ca and Ti in CaTiN2 appear under- and over-bonded (Table 1 and S1), respectively. Such lower valence for alkaline earth site than expected is frequently observed in nitirdes,19 implying inherently subvalent Ca in CaTiN2. Figure 4 shows the temperature dependency of dielectric permittivity εr and dielectric loss tan δ values in 104−106 Hz for the CaTiN2 ceramic. At room temperature, CaTiN2 displays high εr values ∼1300−2500 within 104−106 Hz. The εr in 104− 106 Hz of CaTiN2 ceramic shows weak temperature dependency within 80−450 K: the permittivity at 106 Hz changes from 1400 at 80 K to 1550 at 450 K, showing temperature coefficient of permittivity ∼290 ppm/K. The dielectric loss of CaTiN2 ceramic is dependent on frequency: at 106 Hz the tan δ values are less than 0.07 within 80−450 K, which are lower than those (∼0.2−0.3) of the high-permittivity oxynitrides SrTaO2N and BaTaO2N.12 Complex impedance plots (Figure 5) of the CaTiN2 ceramic display single semicircular arcs at each temperature, which perfectly start from the origin, i.e., they do not have nonzero
Figure 2. Rietveld plot of NPD data for CaTiN2. The two rows of vertical marks denote the Bragg diffraction positions of CaTiN2 (bottom) and the minor TiN (top, ∼8.8 wt %) phases.
distorted layered structure for CaTiN2. The NPD data refinement indicates essentially full occupancies on each site, confirming the stoichiometric composition for CaTiN2. Lowering the symmetry down to the polar P21mn or Pm21n slightly improved the fit of NPD data to Rwp = 2.58% (Figure S5). Therefore, the centric and polar symmetries can not be firmly discerned for the ambient temperature structure of CaTiN2. Here we focus on the centric Pmmn symmetry for describing the ambient temperature structure of CaTiN2 (Table 1) whereas the polar structure in P21mn (Table S1−2) will be mainly used for discussion of possible polarity regarding the high bulk εr of CaTiN2 in the later paragraph. Figure 3 shows the orthorhombic structures of CaTiN2 in Pmmn and P21mn and polyhedra of TiN5 and CaN9. Compared with the tetragonal SrTiN2, the square-based pyramid TiN5 in the orthorhombic CaTiN2 is more distorted, showing more pronounced multiple Ti−N bonding distances (Figure 3b): one short apical Ti−N2 bond (∼1.89 Å), and four long basal Ti− N1 bonds (e.g., two ∼1.99 Å and two ∼2.02 Å for Pmmn). 1990
DOI: 10.1021/acs.chemmater.6b05226 Chem. Mater. 2017, 29, 1989−1993
Communication
Chemistry of Materials
S7a)1 as the lower capacitance plateau from the bulk response becomes more dominative to the impedance data in the low temperature region below 200 K (Figure S7b).6 Clearly, the high permittivity of CaTiN2 nitride is not falling into the extrinsic effect case of the CCTO ceramic, given the weakly temperature/frequency-dependent permittivity in high-frequency region 104−106 Hz over a wide temperature range of 80−450 K for the CaTiN2 ceramic. The visible and near-infrared diffuse reflectance spectrum of CaTiN2 (Figure S8) reveals an absorption edge around 955 nm, giving a band gap Eg ∼ 1.3 eV for CaTiN2. The band structure calculation slightly underestimates the band gap of CaTiN2 as 1.165 eV (Figure S9). The experimental and calculated band gaps indicate that CaTiN2 is a direct semiconductor, coinciding with the leaking insulator behavior of CaTiN2 (bulk conductivity σb varies within 10−5−10−6 S/cm within 270− 450 K in Figure S10) with an activation energy Ea ∼ 0.88 eV. The total and partial density of states (DOS) (Figure S11) indicate that the valence and conduction bands are mainly composed of the N 2p orbitals and Ti 3d orbitals in CaTiN2, respectively. Variable-temperature (VT) XRD data of CaTiN2 collected from 120 to 1273 K (Figure S12) under vacuum indicate that the CaTiN2 phase is stable without showing apparent phase transition evidence below 773 K and starts to be oxidized when the temperature is warmed up above 773 K. Using the reducing mixed H2−N2 gas in the VT-XRD high-temperature furnace did not prevent the oxidation of CaTiN2. Refined cell parameters of CaTiN2 below 773 K indicate anisotropically thermal expansion (Figure 6): the a-axis shows contraction with increase of
Figure 4. Temperature dependency of Permittivity εr and dielectric loss tan δ (inset) of CaTiN2 ceramic.
Figure 5. Selected complex impedance plots for the CaTiN2 ceramic within 180−450 K. The inset enlarges the high-temperature data within 350−450 K, showing that theses semicircular arcs start from the origin in the high-frequency region.
intercept at high frequency. Therefore, these arcs could be ascribed to the bulk responses. The semicircular arc can be modeled with a parallel RC circuit: the intercept of the semicircular arc at low frequency is estimated as R; the associated C value calculated from the equation 2πf maxRC = 1 ( f max is the frequency corresponding to the maximum imaginary impedance Z″max) is 2.3 × 10−10 F/cm. This value is close to that (2.2 × 10−10 F/cm) estimated from the maximum imaginary modulus M″max using the equation ε0/ (2M″max) (ε0 is the capacitance of free space, 8.854 × 10−14 F/ cm).23 This indicates that the Z″max peak is associated with the same RC elements as the modulus peak (the frequencies corresponding to Z″max and M″max are close in Figure S6); thus the semicircular arc can be ascribed to a single bulk component response showing large capacitance ∼10−10 F/cm (i.e., highbulk permittivity). In other words, the high εr of CaTiN2 is an intrinsic bulk property, unlike the extrinsic IBLC effect in the CCTO ceramics. The high permittivity from the IBLC effect is highly dependent on temperature and frequency below room temperature because the bulk response becomes dominative in the dielectric behavior on cooling given its enhanced resistivity at low temperature: the permittivity of the CCTO ceramic in high-frequency region (105−106 Hz) displays dramatic dropping down to ∼100 from 106 within 150−50 K (Figure
Figure 6. Temperature dependency of cell parameters for CaTiN2.
temperature; whereas the b and c axes display thermal expansion leading to normal thermal expansion on volume (Figure S13). The contraction of the a-axis and expansion of the b-axis of the CaTiN2 decrease the length gap between a and b axes (Figure 6), indicating that with the increase of temperature CaTiN2 could transform into a SrTiN2-like tetragonal phase with equal a and b axes if it was stable at elevated temperatures. As demonstrated by the impedance spectroscopy data, the high εr (>103) behavior is intrinsic bulk property for the CaTiN2 nitride. Here the origins for the high bulk εr are discussed in term of the possible polar distortion and the uneven coordination environment in the structure of CaTiN2. Although the structural analysis and SHG measurement did not firmly support (neither exclude) the polar symmetry for CaTiN2 and the ferroelectric hysteresis measurement on the 1991
DOI: 10.1021/acs.chemmater.6b05226 Chem. Mater. 2017, 29, 1989−1993
Communication
Chemistry of Materials Notes
CaTiN2 ceramics was not successful owing to the leaking property of CaTiN2, the lower global instability index24 (GII) ∼ 0.31 and energy (−6306.36 eV) for the polar structure than those (∼0.37 and −6306.34 eV) for the centric structure indicate that the polar structure has a slight preference over the centric structure. In addition, the refined anisotropic atomic displacement parameters (ADPs) show slightly anisotropic displacement for Ca atoms along the a-axis in the centric structure but essentially isotropic displacement for Ti atoms (Table S3), agreeing respectively with the large and small placements of Ca (∼0.13 Å) and Ti (∼0.025 Å) along the aaxis described above in the polar structure. It is likely that the average structure of CaTiN2 is in P21mn but there exists a substantial local disorder. The spontaneous polarization (Ps)25 in CaTiN2 calculated using the refined polar structure of CaTiN2 (Table S1) is 49 μC/cm2, compared to that (75 μC/ cm2) for PbTiO3.26 This indicates that the polar CaTiN2, if existed, actually could have potential to achieve large permittivity. On the other hand, the unevenly (i.e., asymmetrically) 5- and 9-coordinated polyhedral environments in CaTiN2 structure itself might be more beneficial for producing strong electric dipoles in electric field than the evenly coordinated environment (e.g., 6/12-coordinate polyhedra in perovskite). Coupled with both large polarizability for the N3− ligands and the orthorhombic distortion, this unevenly coordinated network in CaTiN2 could result in high bulk εr even without spontaneous polarization. This could stimulate further theoretical modeling and exploitation of new dielectrics in line with the structural feature in CaTiN2. In summary, a new KCoO2-type layered nitride CaTiN2 in an orthorhombic distorted structure was successfully synthesized and weakly temperature-dependent high bulk εr ∼ 1300−2500 at 104−106 Hz within 80−450 K is demonstrated on this layered CaTiN2 nitride ceramic, emphasizing great opportunity for discovery of the high-bulk-εr dielectrics from (oxy)nitrides given the higher polarizability of N3− than O2− and the complex and flexible structural chemistry related to N3−.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS National Natural Science Foundation of China (No. 21371015 and 21622101), Program for New Century Excellent Talents in University (No. NCET-13-0752) and Guangxi University Hundred Talent Program for Returned Scholars are acknowledged for the financial support. Prof. Jiyong Yao (Technical Institute of Physics and Chemistry, CAS) and Mr. Kun Lin (USTB) are acknowledged for the help on the SHG effect measurement and spontaneous polarization calculation, respectively.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05226. Synthesis, characterization and calculation methods and miscellaneous data of CaTiN2 on EDS, TGA, morphology, Rietveld fits of PXRD and NPD data, VT-XRD, conductivity, comparison with CCTO ceramic, diffuse reflection spectrum, band structure and total/partial DOS, structural parameters in P21mn (PDF) Crystallographic information for CaTiN2 in Pmmn and P21mn (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*(X.W.) E-mail:
[email protected]. *(X.K.) E-mail:
[email protected]. ORCID
Xiaojun Kuang: 0000-0003-2975-9355 Jun Chen: 0000-0002-8693-2508 Xianran Xing: 0000-0003-0704-8886 1992
DOI: 10.1021/acs.chemmater.6b05226 Chem. Mater. 2017, 29, 1989−1993
Communication
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DOI: 10.1021/acs.chemmater.6b05226 Chem. Mater. 2017, 29, 1989−1993
