Top-Seeded Single-Crystal Growth, Structure, and Physical Properties

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Top-Seeded Single Crystal growth, Structure, and Physical Properties of Polar LiCrP2O7 Elise Pachoud, Weiguo Zhang, Joshua H. Tapp, Kao-Chen Liang, Bernd Lorenz, Paul Ching-Wu Chu, and P. Shiv Halasyamani Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 01 Nov 2013 Downloaded from http://pubs.acs.org on November 11, 2013

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Crystal Growth & Design

Top-Seeded Single Crystal growth, Structure, and Physical Properties of Polar LiCrP2O7 Elise Pachoud1†, Weiguo Zhang1†, Joshua. Tapp1, Kao-Chen Liang2, Bernd Lorenz2, Paul C. W. Chu2, P.Shiv Halasyamani1,3 1

Department of Chemistry, University of Houston, 136 Fleming Building, Houston TX 77204, United

States 2

Texas Center for Superconductivity and Department of Physics, University of Houston, Houston

Science Center, Houston TX 77204, United States 3

Department of Chemistry, Aalto University, Kemistintie 1, 02150 Espoo, Finland

†These authors contributed equally. CORRESPONDING AUTHOR EMAIL ADDRESS： [email protected], [email protected]

Abstract: Single crystals of polar LiCrP2O7 were grown by using a top-seeded solution growth method. In addition to the crystal structure, detailed magnetic measurements reveal anisotropic magnetic behavior with a spin-flop transition. Piezoelectric, polarization, and heat capacity measurements were also measured on the single crystals. Powder second-harmonic generating measurements, using 1064 nm radiation, revealed type 1 phase-matching behavior with an efficiency of approximately 30 × α-SiO2.

1. Introduction ACS Paragon Plus Environment

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Phosphate materials, with their structural network based on PO4 tetrahedra, that can be isolated or corned-shared forming PnO3n+1 entity, have various interesting properties. One can cite applications in non linear optics for the noncentrosymmetric KTiOPO41,2 or in electrochemistry with ionic conductivity in phosphates having the NASICON structure,3 or in the cathode materials LiMPO4.4,5 Magnetic properties can also be encountered in transition metal phosphates. Among them, the diphosphates AMP2O7 are interesting to study as their magnetic behavior is ruled only by super-super-exchange (SSE) interactions – through M-O...O-M paths – involving two oxygens of a phosphate polyhedron. There are no simple rules for super-super-exchange interactions. Recent theoretical studies have shown that the strength and sign of the SSE interaction are mainly determined by the O…O distance6 and the relative angle of the polyhedra involved in the exchange.7 Thus, these compounds are good examples to study what govern the sign and strength of the SSE interactions, as they are often neglected in materials having both super-exchange (SE) and SSE interactions.6, 8 Moreover, when A is a lithium cation, the structure is reported non centrosymmetric (space group P21). The possible coexistence of ferroelectricity and magnetic ordering makes the LiMP2O7 compounds good candidates for multiferroicity. Indeed, the recent growth of large single crystals of LiFeP2O7 9 has allowed the detailed study of the magnetoelectric effect predicted in this material by Nénert.10 The compound is polar but not ferroelectric at room temperature, and a weak ferromagnetic component has been measured along b at TN = 27 K. At this temperature, a significant magnetoelectric effect is observed by a sharp peak in the pyroelectric current, attributed to a drop of the polarization below TN. 11 Similar interactions can be expected in the chromium analogue. LiCrP2O7 has been reported isostructural, and a magnetic ordering, yet to be measured, is expected by the presence of Cr3+ (3d3). However the structure report has been done using powder X-ray diffraction, by Rietveld refinement starting from the parameters of the assumed isotypic LiMnP2O7.12 As for the functionality of this material, only one study on Li intercalation / deintercalation has been published,13 and even though the magnetic properties of the solid solution LiCryFe1-yP2O7 has been studied, the authors did not report the data for y = 1.14 ACS Paragon Plus Environment

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These considerations have motivated a complete study of this compound. LiCrP2O7 has been synthesized and characterized in polycrystalline and in single crystal forms. In this manuscript, we report for the first time the crystal structure from single crystal X-ray diffraction, and the dielectric properties as well as the magnetic behavior. Growth of large single crystals by the TSSG method has allowed us to investigate the anisotropy of these properties.

2. Experimental Section The polycrystalline sample was prepared by solid state reaction. 1 g of a stoichiometric mixture of LiH2PO4 (Alfa Aesar, 97%), NH4H2PO4 (Alfa Aesar, 98.0%) and CrCl3.6H2O (Acros, 98%) were ground, pressed in one pellet and heated at 200 °C for 5 h in an alumina crucible in air to decompose the precursors. The obtained powder was then calcinated at 700 °C for 20 h and 950 °C for 20 h with intermediate grinding and pelletizing. Single crystals of LiCrP2O7 were grown by the flux method. Polycrystalline LiCrP2O7 was mixed thoroughly with LiH2PO4 and NH4H2PO4with the molar ratio LiCrP2O7 : LiH2PO4 : NH4H2PO4 = 5 : 26 : 1. The mixture was heated to1100 °C in a platinum crucible in a vertical furnace equipped with a PtRh/Pt thermocouple and an Al-808P controller. The temperature was held for 20 h in order to form a homogenous melt. Once a homogeneous melt had formed, a piece of platinum wire held on an alumina rod was dipped into the melt. The melt was then cooled at a rate 5 °C/h. At 950 °C, small crystals spontaneously nucleated on the platinum wire. These small crystals were carefully extracted and confirmed to be single phase by powder X-ray diffraction. Then the exact saturation temperature was determined (~ 975 °C) by observing the growth or dissolution of crystal seed when soaking in the melt. After several growth runs with a seed crystal, a crystal of sufficient size was obtained to cut as a crystal seed. In order to obtain a large and high quality single crystal, a seed was introduced into homogenous melt with a rotating rate of 10 - 15 rpm at 2 °C higher than the saturation temperature, followed by decreasing the temperature to the saturation point in 30 minutes. From the saturation temperature, the

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melt was cooled at a rate of 0.2 °C per day to about 1°C below the saturation point. After 5 days growth, an as-grown single crystal was then hung above the melt surface and cooled slowly to room temperature (See Figure S1). Powder X-ray diffraction (XRD) patterns were recorded on polycrystalline and ground single crystals with a PANalytical X’Pert Pro diffractometer equipped with a Cu Kα radiation (λ = 1.54056 Å) in the 570° 2θ range for rapid scans, and 5-100° for high quality scans. A green block-shaped crystal (0.4 × 0.4 × 0.2 mm3) was used for single-crystal structure determination. Data were collected on an APEX II CCD detector Bruker instrument using graphite–monochromated Mo Kα radiation. A hemisphere of data was collected using a narrow-frame method with scan widths of 0.30° in ω and an exposure time of 30 s per frame. The data were integrated using the SAINT program (Bruker, v. 4.05) with the intensities corrected for Lorentz polarization, air absorption, and absorption attributable to the variation in the path length through the detector faceplate. Multiscan (from WinGX 15) was used for the absorption correction on the hemisphere of data. The data were solved by direct method using SHELXS-97 and refined using SHELXL-97.16 All of the atoms were refined with anisotropic thermal parameters and converged for I > 2σ(I). All calculations were performed using the WinGX crystallographic software package.15 Thermogravimetric and differential thermal analysis (TG/DTA) were performed on an EXSTAR TG/DTA 6300 thermal analysis system (SII Nano Technology Inc.). 13.87 mg of polycrystalline LiCrP2O7 were placed in a Pt pan and successively heated and cooled at 10°C/min from room temperature to 1300°C in N2 atmosphere. The residue was then collected and checked by powder XRD. Infrared spectra were recorded on a Perkin Elmer Spectrum 100 FT-IR spectrometer using a Pike MIRacle micrometer pressure clamp. UV-visible reflectance data were collected on a Perkin Elmer LAMBDA 1050 scan UV-vis-NIR spectrophotometer over the 200 – 2000 nm spectral range at room temperature. Poly-tetrafluoroethylene was used as a reference material. The reflectance spectrum was converted to absorbance using the Kubelka-Munk function. 17, 18 Powder Second Harmonic Generation (SHG) experiments were done on a 4 ACS Paragon Plus Environment

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modifier Kurtz-NLO system

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using a pulse Nd:YAG laser with a wavelength of 1064 nm. The

equipment and methodology has been described in ref 20. SHG efficiency has been measured on ground crystals with different particle size ranges: < 20, 20-45, 45-63, 63-75, 75-90, and 90-125 µm. Relevant comparisons with known SHG materials were made by grinding and sieving crystalline α-SiO2 into the same particle size ranges. No index matching fluid was used in any of the experiments. The direct piezoelectric coefficient was measured with a YE2730A d33 meter (APC international, Ltd.). The sample used was a Y-cut single crystal with the dimension 2 × 3 × 1 mm3, with its larger faces perpendicular to the b axis according to the IEEE standard.

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The Y-cut crystal used in the

piezoelectric measurements was also used in the polarization measurements. The polarization loop was measured at room temperature on a Radiant Technologies RT66A Ferroelectric Test System with a TREK high voltage amplifier. Different attempts were performed under a static field of 600-1200 V/cm at 50 Hz. The magnetic properties of the polycrystalline sample were measured in a Physical Property Measurement System (PPMS) Quantum Design cryostat with the VSM option. Magnetization dependence on temperature were measured from 2 to 300K in various magnetic field with zero field cooling (ZFC) and field cooling (FC) processes, and the dependence on magnetic field were measured from -5T to 5T at 2K and 10K. To investigate the magnetic anisotropy and to reveal more specific information about the magnetic order, measurements were carried out in a Quantum Design superconducting quantum interference device (SQUID) magnetometer on an oriented single crystal along three orthogonal directions: the crystallographic axis a and b, and perpendicular to the (a,b) plane, i.e. the c* axis. The magnetic field was varied from 0 to 50 kOe at different temperatures for the M(H) measurements, and the temperature was scanned between 2 and 10K at different magnetic fields for the χ(T) measurements. The heat capacity was measured using the relaxation method implemented in the PPMS. The temperature dependence of the polarization change below room temperature was recorded using the pyroelectric current method in a PPMS. The data was collected along the polar axis (i.e. the b axis) ACS Paragon Plus Environment

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of a single crystal contacted with silver paste, upon cooling and heating at a rate of 3 K/min. The value for the polarization ∆P is obtained by integrating the pyroelectric current, measured by a Keithley 6517 electrometer, and dividing by the contact area.

3. Results and Discussion 3.1. Crystal structure. The crystal structure of LiCrP2O7 has been resolved and refined using single crystal XRD. The crystallographic data are summarized in Table 1. The compounds crystallizes in the monoclinic space group P21, thus isostructural to LiFeP2O7,22 and in agreement with the structure report from X-ray powder diffraction.12 In this space group, all atoms are on the only possible Wyckoff position 2a. The refined atomic parameters are listed in Table 2 and a selection of bond distances is presented in Table 3. The structure consists of a three-dimensional [CrP2O7]∞ framework built up of CrO6 octahedra linked with diphosphate groups through their corners and edges (see Figure 1). The octahedrally coordinated Cr 6+

are subject to a small second-order Jahn-Teller distortion,23-29 that results in two 'long' (1.999(2) and

1.992(3) Å), two 'normal' (1.977(3), and 1.976(2) Å), and two 'short' (1.966(3) and 1.945(2) Å) Cr-O bonds (see Figure S2a). The conformation of the diphosphate group is close to eclipsed with a P-O-P angle of 127.77(16) ° and an average dihedral angle O-P..P-O of 9.89 °. In each polyhedron, the P-O distance with the bridging oxygen, 1.600(3) Å for P(1) – O(7), and 1.608(2) Å for P(2) – O(7), is longer than the three bonds (1.519(2), 1.516(2), and 1.517(2) Å for P(1) – O, and 1.516(3), 1.519(3), and 1.504(3) Å for P(2) – O, respectively) with oxygen atoms coordinated to chromium, as expected in a diphosphate group (see Figure S2b).30 The lithium cations are located inside the channels formed by the [CrP2O7]∞ framework. The bond valence sum (BVS) calculations31, 32 for Li+, P(1)5+, P(2)5+, and Cr3+ resulted in values 0.86, 4.97, 5.00, and 2.91, respectively (see Table 4). As space group, P21, is polar, it is relevant to discuss the structural origin of the polarity. As mentioned above, there are three different kinds of distorted polyhedra, CrO6 octahedra, and PO4 tetrahedra (see Figure 1). The arrows in Fig. 1 indicate the approximate directions of the local dipole ACS Paragon Plus Environment

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moments on each kind of polyhedron. The calculated dipole moments33, 34 are 1.96, 2.45, and 2.97 Debye for the CrO6 octahedra, P(1)O4, and P(2)O4 tetrahedra, respectively (see Table 4). However, as seen in Fig. 1, dipole moments from different polyhedra are canceled along the a and c axis directions and partially canceled along the b axis direction, that results in a weak net dipole moment along the b axis direction. This suggests that the measured physical properties related to polarization of LiCrP2O7 will be fairly small. The structural resolution from single crystal X-ray diffraction confirms that LiCrP2O7 is isostructural to other LiM3+P2O7 compounds, M = Sc, V, Mn, Fe, Mo, In.22, 35-39 The size of the akali metal A has been reported to dictate the crystal structure of AFeP2O7 compounds with three possible structures; the main difference being the conformation of the diphosphate groups.40 In this nomenclature, LiCrP2O7 crystallize in the structure III, and thus confirm that the chromium analogues ACrP2O7 follow the same structural evolution with A.

3.2. Physico-chemical characterizations. In the search for crystal growth conditions, TG/DTA measurements were performed between room temperature and 1300 °C. In this temperature range, there is no melting or phase transition as seen in the DTA curve in Fig. 2. The drop in mass at 1200 °C in the TGA plot indicates that the compound starts to decompose. This is confirmed by the appearance of small peaks of Cr2O3 in the XRD pattern of the residue, in addition to those of LiCrP2O7. The phase does not melt congruently, hence necessitate the use of a flux for the crystal growth. The infrared spectrum in the 1400-600 cm-1 range (See Fig. S3) can been indexed following the previous studies made on LiCrP2O7.13,

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The P2O7 group has the symmetry C2v, therefore has 17

infrared active vibrations.41 The asymmetric stretching vibrations of the terminal PO3 bonds display three bands at high frequencies (1238 (s), 1138 (vs) and 1096 (vs) cm-1), and two bands (1074 (vs) and 1042 (w) cm-1) are associated with the symmetric stretching vibrations. The presence of the P-O-P bond is illustrated by the presence of the asymmetric (968 (s) and (953 (s) cm-1) and symmetric (772 cm-1) stretching vibrations of the P-O-P bridge. At lower frequencies, i.e. in the 700-600 cm-1 range, bands can ACS Paragon Plus Environment

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be assigned to the binding vibrations of the terminal PO3 bonds, with the exception of the strong band at 628 cm-1 that belongs to the stretching mode of the Cr-O bonds in a CrO6 octahedron.42 The UV-vis diffuse reflectance spectra, as shown in Figure S4, indicates the absorption energy for LiCrP2O7 is approximately 1.6 eV. Absorption (K/S) data were calculated from the Kubelka-Munk function.18 F ( R) =

(1 − R ) 2 K = 2R S ,

Where R represents the reflectance, K is the absorption coefficient, and S is the scattering factor. In a K/S versus E (eV) plot, extrapolating the linear part of the rising curve to zero provides the onset of absorption at 1.6 eV (See Figure S4). There are some broad bands in the range of 2 ~ 6 eV attributable to the d-d transitions of Cr, and Cr to ligand charge transfer.43 The onset of bands indicate the UV absorption edge of LiCrP2O7 single crystal is around 775 nm. The crystal structure of LiCrP2O7 being noncentrosymmetric and polar, powder SHG and dielectric measurements on single crystals are of interest and have been performed at room temperature. The dependence of the SHG efficiency with the particle size in the 10 - 125 µm range is displayed in Fig. 3. The evolution is typical for a type I phase-matchable, but the intensities are low. In the 45 - 63 µm particle size range, the SHG efficiency is about 30 × α-SiO2; a value to be compared to 200 × α-SiO2 for the isostructural LiFeP2O7.9 However, the intense green light of the second harmonic is clearly visually observed during the measurements. It is likely that the lower SHG efficiency of LiCrP2O7 compared with LiFeP2O7 is attributable to the absorption properties of the former. Piezoelectric measurements were performed on a Y-cut crystal by the direct method. The d22 piezoelectric coefficient, i.e. the polarization along the b axis induced by the mechanical force applied in the same direction, is measured at 1.5 pC/N. This value is in the same range as the d22 coefficients for LiFeP2O7, 1.2 pC/N and 1.9 pm/V measured by the direct and converse methods respectively.9 For LiCrP2O7, the sample was too small to be used with the converse method.


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Attempts at measuring ferroelectric hysteresis loops resulted in a linear dependence of the polarization with the electric field (see Figure S5), indicating that LiCrP2O7 is not ferroelectric. In other words, the polarization cannot be switched under an external electric field. To understand this irreversibility, the local polarizations in the materials need to be examined. The polarity in LiCrP2O7 is attributable to the local dipole moments observed in the CrO6 octahedra and PO4 tetrahedra. For the materials to exhibit ferroelectric behavior, the polarization must be ‘switchable’, or ‘reversible’ in the presence of an external electric field. For d0 transition metals in octahedral oxide coordination environments, it is possible for the cation to be ‘switched’ from one corner, edge, or face to the opposite, for example, BaTiO3 (corner) and LiNbO3 (face),44,

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in the presence of an external electric field resulting in

ferroelectric behavior. In LiCrP2O7, infinite [CrP2O11]∞ chains are observed that share corners, in twodimensions, to create the three-dimensional framework. The reversal in the macroscopic polarization (see Table 4) would require substantial rearrangements of the polyhedra, as well as metal-oxygen bond breaking. Thus it is suggested that polarization reversal is highly energetically unfavorable, and therefore LiCrP2O7 is not ferroelectric.

3.3. Magnetism. The magnetic properties were first investigated on the polycrystalline sample. The temperature dependence of the magnetic susceptibility χ measured at 500 Oe present identical ZFC and FC curves with a sharp peak – attributed to an antiferromagnetic transition – at TN = 6 K (Fig. 4). The Curie-Weiss temperature extracted from the fit in the paramagnetic region is estimated at θCW = -2.6 K, revealing weak antiferromagnetic interactions, and the effective moment µeff = 3.78 µB is close to the expected value for the spin only Cr 3+ 3.87 µB in the high-spin state. The magnetic properties of the solid solution LiFe1-xCrxP2O7 has been studied recently:14 the chromium substitution for iron causes the decrease in the transition temperature, as well as in | θCW | and µeff. The difference between ZFC and FC curves is also reduced. The authors interpret these results by a breakdown of magnetic ordering; however, they did not detect the antiferromagnetic order in the non-substituted LiCrP2O7. Their results,


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together with the antiferromagnetic order found in LiCrP2O7 (see above), can be understood as an evolution from the weak ferromagnetism of LiFeP2O7 to the antiferromagnetism of LiCrP2O7. The magnetic properties of the iron analogue are highly anisotropic, with the weak ferromagnetic moment measured along the b axis originating from a canted antiferromagnetic order.11 Magnetic measurements were carried out on an oriented crystal of the chromium analogue to see if a similar behavior is present. The susceptibility curves along a and b are nearly identical (Fig. 5). Decreasing in temperature, the χ values increase with a sharp maximum at TN, and then are nearly temperature independent below TN, forming a plateau after the transition. The susceptibility data measured with the field applied along c* show a significantly different temperature dependence, sharply decreasing below TN to the lowest temperature (Fig. 5). The strong uniaxial anisotropy of the magnetic susceptibility indicates a preferred orientation of the Cr spins with the spin easy axis aligned with the c* orientation in the crystal. It was shown for various spin models (see for example the Ising model with transverse field

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) that in the

antiferromagnetic state the transverse (with respect to the spin orientation) susceptibility is constant below TN whereas the longitudinal susceptibility decreases strongly with decreasing temperature below the antiferromagnetic transition. Contrary to the related compound LiFeP2O7, no ferromagnetic response could be detected along any of the three orthogonal orientations (a, b, c*). Therefore we conclude that the magnetic order is antiferromagnetic with the spins aligned along the c* axis and no spin canting is present which could result in a weak ferromagnetic moment. This makes the magnetic order in LiCrP2O7 fundamentally different from the magnetic structure of LiFeP2O7. In the latter compound, the Fe spins are mainly aligned with the a-axis and only slightly tilted towards c* with the small ferromagnetic canting along b.11, 47 In high magnetic fields the Néel temperature is reduced, as expected for an antiferromagnetic state. The magnetizations M/H along a and b are shown in Fig. 6a for different field values. Both data sets are very similar (note that the b-axis data are vertically offset in the figure for better clarity) and the shift of TN in both field orientations is identical, consistent with the uniaxial nature (preferred c*-axis) of the magnetic ACS Paragon Plus Environment

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order. Fig. 6b shows the high-field magnetization along the c*-axis. Remarkably, the magnetization below TN is suddenly enhanced in fields above 15 kOe and is almost independent of temperature in the high-field regime. This indicates a spin-flop transition induced by the field applied along the c*-axis. The suppression of TN with H || c* is slightly larger than H || a, b. The transition into the high-field (spin-flop) phase is seen in more detail in the magnetization vs. field data of Fig. 7. With the field orientation along c* a clear metamagnetic transition is observed at 17 kOe (Fig. 7a). With increasing temperature, the critical field of this transition shows a minute decrease. In contrast, magnetic fields along a and b cause the usual linear increase of M at low fields and weak anomalies at higher fields are due to the transition from the antiferromagnetic into the paramagnetic phase (Fig. 7b). The similarity of both data sets with fields along a and b underline the uniaxial nature of the Cr spins in the ordered phase. With the temperature and field dependent magnetization data of Figs. 5 to 7, a phase diagram can be constructed for LiCrP2O7, as shown in Fig. 8. Here different colors/symbols of the phase boundaries refer to the three orientations of the magnetic field, a, b, and c*. In transverse fields, H || a and H || b, TN decreases continuously and at the same rate with H and the phase boundary can be extrapolated to zero temperature yielding a critical field of about 60 to 70 kOe. The low-temperature phase is antiferromagnetic. In longitudinal fields, however, the antiferromagnetic phase is separated into two regions by the spin-flop transition. The origin of the spin-flop is a gain in magnetic exchange energy of an antiferromagnetic spin system above a critical longitudinal field when the spins rotate by about 90

o

positioning themselves perpendicular to the field with a small canting along the field direction. This happens in systems with uniaxial spin anisotropy in longitudinal magnetic fields. The spin-flop observed in LiCrP2O7 supports our earlier conclusion that the Cr spins at zero field are aligned with the crystal’s c*-axis. LiCrP2O7 is thus very different from LiFeP2O7. There is no ferromagnetic component along the b axis, but a spin-flop metamagnetic transition occurs along c*. The thermodynamic properties at the transition into the antiferromagnetic state are described by the heat capacity Cp(T), shown in Fig. 9. The sharp λ-shaped peak at TN is the signature of a second order ACS Paragon Plus Environment

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phase transition, similar to the heat capacity peak in LiFeP2O7.11 The magnetic contribution to the heat capacity, Cpmag(T) shown in the upper inset of the figure, is obtained by subtracting the lattice part which is approximated by a linear extrapolation of Cp/T vs. T2 (Debye theory). The magnetic entropy Smag is calculated by integrating Cpmag(T)/T and it is shown in the lower inset of Fig. 9. At 25 K, Smag approaches 95 % of R ln4 (R universal gas constant), which is the entropy value of a completely disordered spin 3/2 system. This confirms the result derived from high-temperature magnetization measurements (Fig. 4) that the Cr 3+ ion is in the high-spin S = 3/2 state. At the Néel temperature, nearly 80 % of the maximum entropy value is obtained indicating that spin fluctuations above TN are not significant. 3.4. Electrical polarization and magnetoelectric effect. Since Even though LiCrP2O7 and LiFeP2O4 are not ferroelectric at room temperature, they are both polar, and LiFeP2O7 is magnetoelectric with a sharp peak in the pyroelectric current at TN along the polar axis b.11 LiCrP2O7 could thus be a good candidate for such effect: it is isostructural to LiFeP2O7, magnetically ordered at TN = 6 K, and additionally undergoes a metamagnetic transition, a feature frequently observed in multiferroic compounds: mainly in type-II multiferroics, MnWO4,48 TbMnO3, 49 and CuFeO2,50 but also in type-I like BiFeO3.51 These considerations have motivated pyroelectric measurements. The change of the electrical polarization ∆P(T) measured below room temperature along the crystal’s polar b-axis is shown in Fig. 10 for both compounds. ∆P(T) of LiCrP2O7 is smaller than that of LiFeP2O7, reaching about 60 % of the maximum value of the latter compound. More significantly, no measurable change of ∆P(T) could be detected at the Néel temperature of LiCrP2O7, unlike in LiFeP2O7. Furthermore, no magnetoelectric effect (change of polarization with external magnetic fields) was found in LiCrP2O7. The missing magnetoelectric properties of LiCrP2O7 are most likely due to the differences in the magnetic orders. Although a coupling of the b-axis polarization to the antiferromagnetic order parameter is allowed by symmetry of the monoclinic space group P21 (see discussion in Ref. 11), the coupling must be very weak so that it was not detected experimentally. It should be noted that the magnetoelectric effect observed in LiFeP2O7 was attributed to the weak ferromagnetic moment which is 12 ACS Paragon Plus Environment

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missing in LiCrP2O7. LiCrP2O7 is thus not magnetoelectric or the effects are so small that they could not be resolved in the current experiments.

4. Conclusions We have synthesized LiCrP2O7 in ceramic and single crystal forms. Single crystal X ray diffraction was used to confirm the crystal structure with the polar space group P21. But this polarization is not reversible, thus the material is not ferroelectric. The magnetic measurements on the polycrystalline sample revealed an antiferromagnetic ordering below TN = 6 K, and a metamagnetic transition at ≅ 17 kOe. The growth of large single crystals using TSSG technique has enabled us to study in more details this behavior. The magnetic behavior is anisotropic, as in the LiFeP2O7 analogue, with a spin-flop transition when the magnetic field is applied perpendicular to the (a,b) plane, suggesting that this axis is the easy magnetization direction along which the magnetic moments lie (anti)parallel at low field. The phase transition is of second order as confirmed by heat capacity measurements. The electrical polarization change was measured along the b-axis below room temperature and no anomaly at the magnetic transition temperature was detected. The study reveals the importance of the low-temperature magnetic structure which distinguishes LiCrP2O7 from LiFeP2O7. 5. Acknowledgement. We thank Martin Donakowski and Prof. Kenneth R. Poeppelmeier for diffusereflectance spectra that were obtained at the Keck Biophysics Facility at Northwestern University that is supported by grants from the W.M. Keck Foundation, Northwestern University, the NIH, the Rice Foundation, and the Robert H. Lurie Comprehensive Cancer Center. This work is supported in part by the US Air Force Office of Scientific Research, the T. L. L. Temple Foundation, the J. J. and R. Moores Endowment, and the State of Texas through the Texas Center for Superconductivity at the University of Houston. We also thank Teng-Hao Chen and Prof. Ognjen Miljanic (University of Houston) for the infrared measurements. EP, WZ, and PSH thank the Welch Foundation (Grant E-1457) for support.


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Page 14 of 32

Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org

References (1) Zumsteg, F.C.; Bierlein, J.D.; Gier, T.E. J. Appl. Phys. 1976, 47, 4980-4985. (2) Stucky, G.D.; Phillips, M.L.F.; Gier, T.E. Chem. Mater. 1989, 1, 492-509. (3) Padhi, A.K.; Nanjundaswany, K.S.; Masquelier, C.; Goodenough, J.B. J. Electrochem. Soc. 1997, 144, 2581-2586. (4) Padhi, A.K.; Nanjundaswany, K.S.; Goodenough, J.B. J. Electrochem. Soc. 1997, 144, 1188-1194. (5) Okada, S.; Sawa, S.; Egashira, M.; Yamaki, J.-I.; Tabuchi, M.; Kageyama, H.; Konoshi, T.; Yoshino, A. J. Power Sources 2001, 97-98, 430-432. (6) Whangbo, M.-H.; Dai. D.; Koo, H.-J., Dalton Trans. 2004, 3019-3025. (7) Tsirlin A. A.; Möller A.; Lorenz B.; Skourski Y.; Rosner H. Phys. Rev. B 2012, 85, 014401. (8) Ko, H.-J.; Dai, D.; Whangbo, M.-H. Inorg. Chem. 2005, 44, 4359-4365. (9) Zhang, W.; Halasyamani, P. S. Cryst. Growth Des. 2012, 12, 2127-2132. (10) Nénert, G. Orbital ordering and multiferroics. Ph.D. Dissertation, University of Groningen, 2007. (11) Liang, K.-C.; Zhang, W.; Lorenz, B.; Sun, Y. Y.; Halasyamani, P. S.; Chu, C. W. Phys. Rev. B 2012, 86, 094414. (12) Ivashkevich, L. S.; Selevich, K. A.; Lesnikovich, A. I.; Selevich, A. F. Acta Crystallogr., Sect. E: Struct. Rep. Online 2007, E63, i70-i72. (13) Gangulibabu; Bhuvaneswari, D.; Kalaiselvi, N. Appl. Phys. A: Mater. Sci. Process. 2009, 96, 489493. ACS Paragon Plus Environment

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(14) Bih, H.; Saadoune, I.; Ehrenberg, H.; Fuess, H. J. Solid State Chem. 2009, 182, 821-826. (15) Farrugia, L.J. J. Appl. Crystallogr. 1999, 32, 837-838. (16) Sheldrick, G.M. SHELXS-97, SHELXL-97. University of Göttingen, 1997. (17) Tauc, J. Mater. Res. Bull. 1970, 5, 721-729. (18) Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 12, 593. (19) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798-3813. (20) Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Chem. Soc. Rev. 2006, 35, 710-717. (21) IEEE Standard on Piezoelectricity; IEEE Inc.: New York, 1988. (22) Riou, D.; Nguyen, N.; Benloucif, R.; Raveau, B. Mat. Res. Bull. 1990, 25, 1363-1369. (23) Opik, U.; Pryce, M. H. L. Proc. R. Soc. (London) 1957, A238, 425-447. (24) Bader, R. F. W. Mol. Phys. 1960, 3, 137-151. (25) Bader, R. F. W. Can. J. Chem. 1962, 40, 1164-1175. (26) Pearson, R. G. J. Am. Chem. Soc. 1969, 91, 4947-4955. (27) Pearson, R. G. J. Mol. Struct. (THEOCHEM) 1983, 103, 25-34. (28) Wheeler, R. A.; Whangbo, M.-H.; Hughbanks, T.; Hoffmann, R.; Burdett, J. K.; Albright, T. A. J. Am. Chem. Soc. 1986, 108, 2222-2236. (29) Kunz, M.; Brown, I. D. J. Solid State Chem. 1995, 115, 395-406. (30) Durif, A. Crystal Chemistry of Condensed Phosphates; Plenum Press: New York, 1995. (31) Brown, I. D.; Altermatt, D. Acta Crystallogr. Sect. B. 1985, B41, 244-247.


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Page 16 of 32

(32) Brown, I. D. The Chemical Bond in Inorganic Chemistry. Oxford University Press: New York, 2002, (33) Debye, P. Phys. Z. 1921, 22, 302. (34) Debye, P. Polar Molecules. Chemical Catalog Company: New York, NY, 1929. (35) Vitins, G.; Kanepe, Z.; Vitins, A.; Ronis, J.; Dindune, A.; Lusis, A. J. Solid State Electrochem. 2000, 4, 146-152. (36) Li, K.H.; Wang, Y.P.; Chen, Y.B.; Wang, S.L. J. Solid State Chem. 1990, 86, 143-148. (37) Ivashkevich, L.S.; Delevich, K.A.; Lesnikovich, A.I.; Selevich, A.F.; Lyakhov, A.S. Z. Kristallogr. 2006, 221, 115-121. (38) Ledain, S.; Leclaire, A.; Borel, M.M.; Raveau, B. Acta Cryst. 1996, C52, 1593-1594. (39) Tranqui, D.; Hamdoune, S.; Le Page, Y. Acta Cryst. 1987, C43, 201-202. (40) Belkouch, J.; Monceaux, L.; Bordes, E.; Courtine, P. Mater. Res. Bull. 1995, 30, 149-160. (41) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: Hoboken, NJ, 2007. (42) Brown, D. A.; Cunningham, D.; Glass, W. K. Spectrochim. Acta A-M 1968, 24, 965-968. (43) Miessler, G. L.; Tarr, D. A. Inorgnic Chemistry (Thrid Edition), Pearson Education, Inc. N. J. 2004. (44) Shirane, G.; Danner, H.; Pepinsky, R. Phys. Rev. 1957, 105, 856-860. (45) Abrahams, S. C.; Hamilton, W. C.; Reddy, J. M. J. Phys. Chem. Solids 1966, 27, 1013-1018. (46) Stinchcombe, R. B., J. Phys C: Solid State Phys. 1973, 6, 2459-2483.


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(47) Rousse, G.; Rodriguez-Carvajal, J.; Wurm, C.; Masquelier, C. Solid State Sci. 2002, 4, 973-978. (48) Taniguchi, K.; Abe, N.; Takenobu, T.; Iwasa, Y.; Arima, T. Phys. Rev. Lett. 2006, 97, 097203. (49) Kimura, T.; Goto, T.; Shintani, H.; Ishizaka, K.; Arima, T.; Tokura, Y. Nature 2003, 426, 55-58. (50) Kimura, T.; Lashley, J.C.; Ramirez, A.P. Phys. Rev. B 2006, 73, 220401(R). (51) Tokunaga, M.; Azuma, M.; Shimakawa, Y. J. Phys, Soc. Jpn. 2012, 79, 064713.


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Page 18 of 32

Table 1. Crystallographic data for LiCrP2O7.

a

Chemical formula

LiCrP2O7

M (g.mol-1)

232.88

T (K)

296(2)

Space group

P21 (No. 4)

a (Å)

4.7867(7)

b (Å)

8.0049(11)

c (Å)

6.9093(10)

β (o)

109.003(2)

V (Å3)

250.32(6)

Z

2

ρcalc (g.cm-3)

3.090

µ(mm-1)

2.9

2 θmax (o)

56.68

R (int)

0.0186

GOF (F2)

1.065

R (F)a

0.0178

Rw(Fo2)b

0.0499

Flack param.

-0.02(2)

R(F) = Σ||Fo| − |Fc||/Σ|Fo|. bRw(Fo2) = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2


18

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Refined atomic parameters for LiCrP2O7. The atomic displacement parameters were refined anisotropically, Ueq is defined as one third of the trace of the orthogonalized Uij tensor. Atoms

x

y

z

Ueq2 (Å2)

Li1

0.2973(19)

1.0789(10)

0.3106(12)

0.0270(17)

Cr1

0.71719(11)

0.95198(6)

0.72864(8)

0.00612(19)

P1

0.10126(19)

0.77023(10)

0.48158(12)

0.0067(2)

P2

0.7078(2)

0.67073(10)

1.08604(13)

0.0069(2)

O1

0.3200(5)

0.9140(3)

0.5228(3)

0.0091(6)

O2

0.6288(5)

0.7633(3)

0.8839(4)

0.0100(6)

O3

0.9017(5)

0.7703(3)

0.6128(3)

0.0100(5)

O4

1.1053(5)

1.0184(3)

0.9184(4)

0.0114(5)

O5

0.7527(5)

1.1010(3)

0.5099(3)

0.0094(5)

O6

0.5598(6)

1.1303(3)

0.8541(4)

0.0118(5)

O7

-0.0980(5)

0.8036(3)

0.2492(4)

0.0105(5)


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Page 20 of 32

Table 3. Selected Bond Distances (Å) for LiCrP2O7. Li(1) – O(1)

1.950(7)

Li(1) – O(2 )

2.102(8)

Li(1) – O(3 )

1.966(8)

Li(1) – O(5 )

2.174(9)

Cr(1) – O(1)

1.992(3)

Cr(1) – O(2)

1.977(3)

Cr(1) – O(3)

1.999(2)

Cr(1) – O(4)

1.966(3)

Cr(1) – O(5)

1.976(2)

Cr(1) – O(6)

1.945(2)

P(1) – O(1)

1.519(2)

P(1) – O(3)

1.516(2)

P(1) – O(5)

1.517(2)

P(1) – O(7)

1.600(3)

P(2) – O(2)

1.516(3)

P(2) – O(4)

1.519(3)

P(2) – O(6)

1.504(3)

P(2) – O(7)

1.608(2)


20

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Bond Valence Sum (BVS) and local dipole moment calculations for LiCrP2O7 a.

a

Element

BVS

Local Dipole Moment µ (Debye)

Li

0.86

-

Cr

2.91

1.96

P1

4.97

2.97

P2

5.00

2.45

Please see supporting information for detail calculation.


21


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Page 22 of 32

Figure 1. Crystal structure of LiCrP2O7 in bc plane with indicating local dipole moment directions for different polyhedral. (Cr, P, O, and Li atoms are in blue, purple, red, and green, respectively.).

Figure 2. TG/DTA data of polycrystalline LiCrP2O7.


22

Page 23 of 32


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

23


Figure 3. SHG intensity vs. particle size for LiCrP2O7.

14.5 14.0 13.5

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

13.0 12.5 12.0 11.5 11.0 10.5

0

20

40

60

80

100

120

140

Particle size (µm)


24

Page 25 of 32

Figure 4. Evolution with temperature of the inverse magnetic susceptibility of polycrystalline LiCrP2O7 in 500 Oe. The line corresponds to the Curie-Weiss law fit in the paramagnetic region. Inset: enlargement of the χ(T) curve near TN.

200

160

120

80

−1

χ (mol/emu)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


µeff = 3.78 µB

40

θCW = -2.6 K

0

0

50

100

150

200

250

300

T (K)


25


Figure 5. Low-temperature magnetic susceptibility of LiCrP2O7 measured along three orthogonal axes, a, b, c*. All measurements were conducted at H = 100 Oe with identical data obtained upon zero-field cooling and field cooling.

1.2 a-axis b-axis c*-axis 0.8

-3

χ (10 emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

0.4

0.0

0

4

8

12

16

20

T (K)


26

Page 27 of 32

Figure 6. Magnetization M/H of LiCrP2O7 single crystal at high fields up to 50 kOe. The two data sets in (a) are vertically offset for better clarity.

1.4

(a)

1.3

(b) 1.2

1.2

-1

-1

M/H (emu g kOe )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c*-axis

1.0 1.1 b-axis

0.8

1.0 0.6 0.9 10 kOe 20 30 40 50

0.8 0.7 2

4

6

5 kOe 15 25 35 45 50

0.4 0.2 a-axis

8

10

0.0

2

4

T (K)


6

8

10

T (K)

27


Figure 7. Magnetization vs. field data of LiCrP2O7 single crystal. The two data sets in (b) are vertically offset for better clarity.

(a)

T=2K 3.25 4.5 5 5.75 7

70 60 50

M (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

80

(b)

T=2K 4.5 5.5 7

70 60 50

40

b-axis 40

30 30 c*-axis

20

20

10 0

a-axis

10 0

10

20

30

40

50

0

0

10

H (kOe)


20

30

40

50

H (kOe)

28

Page 29 of 32

Figure 8. Temperature vs. magnetic field phase diagram for LiCrP2O7. The hatched area corresponds to the spin-flop phase with the field applied along the c* direction (AF: antiferromagnetic, PM: paramagnetic)

H || a H || b H || c*

PM

6

AF

5

T (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 AF spin flop phase

3

2 0

10

20

30

40

50

H (kOe)


29


Figure 9. Molar specific heat Cp(T) of LiCrP2O7. Upper inset: Magnetic contribution to Cp. Lower inset: Magnetic entropy change at low temperatures.

30 -1

(J mol K )

30

mag

10

Cp

20

20

-1

-1

-1

25

Cp (J mol K )

0 0

15

5

10

15

20

25 1.0

10

0.8

/(Rln4)

0.6

mag

5

S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

0.4 0.2 0.0 0

0

0

10

20

30

40

5

10

50

15

20

60

T (K)


30

Page 31 of 32

Figure 10. Electrical polarization of LiCrP2O7 (lower curve) in comparison with LiFeP2O7

10

(upper

curve).

3500 3000

LiFeP2O7

TN

2500 2

∆Pb (µC/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000 1500

LiCrP2O7

1000 500 0

0

50

100

150

200

250

300

T (K)


31


For Table of Contents Use Only SYNOPSIS TOC: Single crystal growth, structure, and physical properties of LiCrP2O7 Elise Pachoud1†, Weiguo Zhang1†, Joshua. Tapp1, Kao-Chen Liang2, Bernd Lorenz2, Paul C. W. Chu2, P.Shiv Halasyamani1,3 1

Department of Chemistry, University of Houston, 136 Fleming Building, Houston TX 77204, United

States 2

Texas Center for Superconductivity and Department of Physics, University of Houston, Houston

Science Center, Houston TX 77204, United States 3

Department of Chemistry, Aalto University, Kemistintie 1, 02150 Espoo, Finland

†These authors contributed equally. CORRESPONDING AUTHOR EMAIL ADDRESS： [email protected], [email protected] Single crystals of LiCrP2O7 were grown by using a top-seeded solution growth method. Detailed magnetic measurements reveal anisotropic magnetic behavior with a spin-flop transition. Piezoelectric, polarization, and heat capacity measurements were also measured on the single crystals.

1.4

(a)

1.3

(b) 1.2

1.2

-1

-1

M/H (emu g kOe )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

c*-axis

1.0 1.1 b-axis

0.8

1.0 0.6 0.9 10 kOe 20 30 40 50

0.8 0.7 2

4

6

T (K)

5 kOe 15 25 35 45 50

0.4 0.2 a-axis

8

10

0.0

2

4

6

8

10

T (K)


32

Top-Seeded Single-Crystal Growth, Structure, and Physical Properties

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