A = Cu, Ag; X = S, Se - American Chemical Society

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Crystal Growth and Intrinsic Properties of ACrX(A = Cu, Ag; X = S, Se) without a Secondary Phase Rikizo Yano, and Takao Sasagawa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00037 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016

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Crystal Growth & Design

Crystal Growth and Intrinsic Properties of ACrX2 (A = Cu, Ag; X = S, Se) without a Secondary Phase Rikizo Yano and Takao Sasagawa* Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama, 226-8503, Japan. ABSTRACT. ACrX2 (A = Cu, Ag; X = S, Se) have a layered triangular structure similar to that of delafossites. In addition to a large variety of physical and chemical properties found in centrosymmetric delafossites, the broken inversion symmetry along the c-axis in this system leads to the emergence of additional functionalities such as piezoelectric, pyroelectric, and/or nonlinear optical properties. We found that chemical-vapor-transport (CVT) technique using CrCl3 was the best growth method for ACrX2. In case of A = Cu, however, a standard CVT procedure always produced a certain amount of a secondary phase, identified as the CuCr2X4 spinel, which significantly affected magnetic and transport properties. By a modified CVT technique with appropriate heat treatments, pure single crystals of CuCrX2 were successfully grown. The resistivity of ACrX2 was systematically changed by the combinations of A and X atoms. In contrast to metallic selenides, sulfides were confirmed to be insulators with giant anisotropy (102~103) between the out-of-plane (c-direction) and the in-plane (ab-plane) resistivity. In both of X = S and Se, resistivity for A = Ag was higher than that for A = Cu. The drastic change in resistivity without carrier doping suggests that ionic conductivity and strong electron correlations play an important role in these materials.

Correspondence Takao Sasagawa MSL R3-37, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, JAPAN TEL & FAX: (81)-45-924-5366 [email protected] http://www.msl.titech.ac.jp/~sasagawa/

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Crystal Growth and Intrinsic Properties of ACrX2 (A = Cu, Ag; X = S, Se) without a Secondary Phase Rikizo Yano* and Takao Sasagawa* Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama, 226-8503, Japan. *R.Y. ([email protected]) and T.S. ([email protected]) KEYWORDS: Layered Chromium Chalcogenides, Transport Properties, Magnetic Properties, Anisotropy

ABSTRACT. ACrX2 (A = Cu, Ag; X = S, Se) have a layered triangular structure similar to that of delafossites. In addition to a large variety of physical and chemical properties found in centrosymmetric delafossites, the broken inversion symmetry along the c-axis in this system leads to the emergence of additional functionalities such as piezoelectric, pyroelectric, and/or nonlinear optical properties. We found that chemical-vapor-transport (CVT) technique using CrCl3 was the best growth method for ACrX2. In case of A = Cu, however, a standard CVT procedure always produced a certain amount of a secondary phase, identified as the CuCr2X4 spinel, which significantly affected magnetic and transport properties. By a modified CVT technique with appropriate heat treatments, pure single crystals of CuCrX2 were successfully grown. The resistivity of ACrX2 was systematically changed by the combinations of A and X atoms. In contrast to metallic selenides, sulfides were confirmed to be insulators with giant anisotropy (102~103) between the out-of-plane (c-direction) and the in-plane (ab-plane) resistivity. In both X = S and Se, resistivity for A = Ag was higher than that for A = Cu. The


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drastic change in resistivity without carrier doping suggests that ionic conductivity and strong electron correlations play an important role in these materials.

INTRODUCTION Layered transition metal chalcogenide compounds are widely used for both commercial applications and basic researches in the field of battery chemistry, catalytic chemistry, solid state chemistry, thermoelectric technology, optoelectronic technology, and so on. This is because they have high chemical stability, large variety of electronic properties derived from the quasi-twodimensional electrons, and tunable electronic properties by doping, intercalation, or deintercalation. In particular, 3d transition metal chalcogenides have strongly localized electrons, which lead attractive features such as magnetic ordering and strong electron correlation effects. Delafossites ABO2 show a large variety of phenomena, such as superconductivity (NaxCoO2) [1], transparent conductivity (CuAlO2) [2], large thermoelectricity (Cu(Rh,Mg)O2) [3], negative thermal expansion (CuScO2) [4], luminescence (CuLaO2) [5], and multiferroic properties (CuFeO2) [6]. Monovalent metal chromium dichalcogenides ACrX2 (A = Cu, Ag; X = S, Se) have edge-shared CrX6 octahedral layers forming triangular lattices (Fig. 1). The octahedral layers similar to those of the delafossites are the source of electronic functionalities. The structural diﬀerence between ACrX2 and delafossites ABO2 is the stacking pattern between X-A-X and O-AO layers along the c-axis; ACrX2 form AX4 tetrahedral layers, while delafossites form O-A-O dumbbell layers. Owing to the tetrahedral layers, ACrX2 do not have the space inversion symmetry along the c-axis, which can induce additional useful electronic response such as piezoelectric, pyroelectric, and/or nonlinear optical properties. While AX4 tetrahedral sites are


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ordered at low temperature, two-dimensional ionic conductivity of A+ can take place at higher temperature by hopping to the adjacent empty sites [7]. Therefore, ACrX2 are expected to be an ideal platform to investigate various phenomena.

Figure 1. Crystal structures of ACrX2 (left) and delafossite ABO2 (right). The difference between two structures, i.e. A-site coordination, is highlighted (middle).

In fact, some attractive properties have been reported for polycrystalline ACrX2. At low temperatures, AgCrS2 and CuCrS2 undergo both antiferromagnetic and structural (from the R3m to Cm space group) phase transitions at 42 K and 39 K, respectively [8,9]. The observed magnetoelastic coupling has been discussed in terms of a spin-driven multiferroic phenomenon [10]. Furthermore, it has been pointed out that, at very low temperatures, AgCrS2 has double ferromagnetic spin stripe ordering [8]. Recently, unusual spin correlated dielectric memory effect has been reported in polycrystalline CuCrS2 [11]. Other compounds also have complex spin structures [9]. In particular, CuCrSe2 is suggested to become a spin liquid state [12]. At high temperatures, on the other hand, all compounds ACrX2 undergo a superionic transition around


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400 ~ 650 K [13, 14]. Owing to ultralow thermal conductivity, it has been proposed that CuCrS2, CuCrSe2, and AgCrSe2 are good candidates for high temperature thermoelectric materials [1517]. However, for the same composition of CuCrS2, there have been inconsistent reports for the transport properties in polycrystalline samples ranging from metallic conductivity (ρ ~ 100 mΩcm) [15] to insulating one (ρ ~ 105 Ωcm) [18]. If CuCrS2 is not the insulator, the abovementioned possibility as the multiferroic compound is denied. It is noted that, even in a single crystal, the presence of CuCr2X4 spinel phase as an impurity phase has been reported [19], which seems to be responsible for the controversial transport results. In order to reveal intrinsic properties of ACrX2, the preparation of high quality single crystals and their systematic evaluations are indispensable. In this study, by a modified chemical vapor transport technique (CVT) using the CrCl3 transport agent, we succeeded in growing pure single crystals of ACrX2 (A = Cu, Ag; X = S, Se) without any secondary phase. In order to grow CuCrX2 crystals free from the secondary phase of CuCr2X4 spinel, we find that two key processes, a rapid quenching and a subsequent high temperature treatment of the polycrystalline powders for CVT, are important. Using the obtained pure single crystals, we measured resistivity and its anisotropy of ACrX2 with systematic combinations of A and X. The electrical conductivity of ACrS2 and ACrSe2 were revealed to be insulating with gigantic anisotropy and metallic, respectively. Taking into account of this significant change in conductivity without carrier doping, ACrX2 is categorized as a strongly correlated electron system. Since ACrX2 are a quite unique system in which so many structural and magnetic features as described above coexist, high quality single crystals of ACrX2 available in this study open a new route to investigate interplay between various phenomena.


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EXPERIMENTAL SECTION Single crystals of ACrX2 (A = Cu, Ag; X = S, Se) were grown by a modified chemical vapor transport technique from polycrystalline powders. Polycrystalline ACrX2 was prepared by a solid state reaction as follows. First, mixtures of appropriate amount of high purity elements were sealed in evacuated quartz tubes (φ6 mm × 10 cm). They were sintered at 800 - 1050oC for 40 h. When A = Cu, we performed two additional key treatments for the polycrystalline powders encapsulated in quartz tubes: rapid quenching using cold water instead of slow cooling process and high temperature treatments after the quenching (quenching was also performed at the end of the high temperature treatments). These were indispensable to obtain pure single crystals as described later. Single crystals of ACrX2 were grown using the obtained pure polycrystalline powder with a little amount of CrCl3 as a transport agent. The mixture was sealed in an evacuated quartz tube and was heated for 200 hours in a homemade transparent 2-zone tube furnace, as described in ref. 20. The hotter side was kept at the same temperature as the former solid state reactions, while the colder side was adjusted to make an appropriate temperature gradient (~ 2oC/cm, as a result, the temperature of colder side of the quartz tube was lower by 20 oC than that of hotter side). In a cooling process for A = Cu, rapid quenching from 1000oC to room temperature was also employed. The phase purity was checked by X-ray diffraction (XRD) using Cu-Kα (Bruker: D2 PHASER powder diffractometer with LynxExe 1D-detector), and structural analysis was performed by single crystal XRD using a single crystal diffractometer (Rigaku: XtaLAB mini). The magnetization curves under magnetic fields parallel to the crystal c-axis were measured by using


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a commercial SQUID magnetometer (Quantum Design: MPMS-XL5). Chemical analysis based on X-ray fluorescence (XRF) was performed using an X-ray analytical and imaging microscope (HORIBA: XGT-5000). The resistivity was measured using a homemade apparatus with a closed cycle refrigerator. The in-plane (ab-plane) resistivity was measured by the standard four-probe method, while the out-of-plane (c-direction) resistivity by the two-probe method. In order to obtain ohmic contacts, Au pads were made by sputter deposition, and Au wires were bonded with an Ag paste.

RESULTS AND DISCUSSION Crystal Growth. In this study, in addition to the CVT technique, we attempted several other growth techniques (Bridgman, melt growth, and flux methods). It was found that any crystal growth in the liquid phase (Bridgman and melt growth) did not take place. This would be partially because of high melting temperature (> 1100oC) of this system. Furthermore, we could not find an appropriate flux for the crystal growth. For the CVT technique, I2, S, Se, and CrCl3 was tried to use as a transport agent. Among then, CrCl3 was found to be the best (the speed of the crystal growth was the fastest). In the optimum heating condition with the CrCl3 agent, another growing kinetics like a self-selected vapor growth (SSVG [21]) might occur in addition to the CVT growth. Platelet crystals were grown on the source materials without wall contact similar to the SSVG. In order to obtain the pure single crystals of CuCrX2, special care must be taken for the preparation of the polycrystalline powders and the subsequent CVT growth, which will be described in detail in the next section. As a result, by a small temperature gradient (~2oC/cm), sizable (5 × 5 × 0.3 mm3) platelet single crystals of ACrX2 were obtained.


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The obtained single crystals had metallic luster as shown in the insets of Fig. 2 (a). As shown in Fig. 2 (b), a two-dimensional diffraction pattern from a CuCrS2 single crystal consisted of clear spots. All of them were assigned to CuCrS2. The XRD patterns from the widest face of a single crystal showed only (0 0 3m) peaks, indicating that the face was the ab-plane. The peak shifts corresponding to the change of the c-axis (20.51, 18.71, 21.28, and 19.42 Å for AgCrS2, CuCrS2, AgCrSe2, and CuCrSe2, respectively) were observed, which agreed well with the data in polycrystalline samples. The XRD patterns for all the obtained crystals did not show any halo peak, diffuse scattering, nor enhanced background, indicating that there is no glassy nor amorphous phase in the samples including the ones prepared by quenching from high temperature. With a large temperature gradient (~10oC/cm), on the other hand, long whisker crystals (5 mm) were also obtained as shown in the middle right inset of Fig. 2, indicating that we can control crystal shape by the temperature difference. The chemical compositions of the obtained crystals were determined by using the X-ray analytical microscope. As shown in Fig. 2 (c), the chemical compositions were spatially homogeneous, and their average values were close to the stoichiometry (e. g. CuCr0.99±0.01S1.98±0.06).


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Figure 2. (a)The XRD patterns from the wide surfaces of the obtained ACrX2 single crystals. Inset pictures show the obtained single crystals, and whisker crystals are AgCrSe2. (b) Twodimensional diffraction pattern of a CuCrS2 single crystal obtained from the single crystal XRD. (c) Spatial distribution of the chemical compositions in a CuCrS2 crystal measured by the X-ray analytical microscope.

Impacts of Spinel Secondary Phases and Procedures to Suppress Them. As mentioned in a former study [19], we confirmed that the crystal growth of CuCrS2 was always accompanied by the secondary phase of the spinel CuCr2S4. It was found that even a trace amount of the CuCr2S4 inclusion significantly changed magnetic and transport properties, as shown below. The same was true for CuCrSe2, whereas AgCrX2 were not disturbed with such a problem because of the nonexistence of stable Ag-Cr spinel phases. The secondary spinel phase in the grown crystals was found to decrease with increasing the purity of the polycrystalline samples for CVT. In this study, we found that higher temperature conditions were preferable to obtain pure CuCrX2. In order to prevent the secondary phase growth, we added two extra procedures at the end of the solid state reaction: (1) water-quenching of the polycrystalline samples in the quartz tube at the end of sintering, and (2) high temperature (~1200oC) treatment of them for a few minutes after the quenching. The water-quenching was also performed at the end of the final CVT process. We investigated how the two additional processes affect the obtained single crystals. We prepared CuCrS2 and CuCrSe2 polycrystalline samples for CVT with and without the waterquenching and with and without the subsequent high temperature treatment (the water-quenching at the end of the CVT process was done for all samples). The CuCrX2 crystals prepared without any treatments were dark black in color, while the others had metallic luster. XRD patterns of the


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obtained crystals are shown in Fig. 3. Without the high temperature treatment, the spinel peaks were found at the slightly higher angle than those of CuCrX2: for example, 111 peaks of CuCr2S4 and CuCr2Se4 appeared at 2θ = 32o and 30o, respectively. On the other hand, the other samples showed only the 003m peaks of CuCrX2.

Figure 3. XRD patterns of CuCrS2 (a) and CuCrSe2 (b) crystals with and without the waterquenching and the high temperature treatment processes. The simulated powder XRD patterns are also shown at the bottom. Asterisks indicate the peaks of the impurity spinel phase.

The magnetization measurements reveal that, except for the single crystals grown from polycrystalline powders with both procedures, there are the spinel secondary phase. This can be confirmed by the fact that the spinel compounds CuCr2X4 are ferromagnetic with high Curie temperatures (TC > 300 K), while CuCrX2 are antiferromagnetic. Figs. 4 (a) and (b) show the field dependence of the magnetization of CuCrX2 crystals. In crystals prepared with both the procedures, linear M-H curves expected for pure CuCrX2, were observed. On the other hand, clear ferromagnetic signals were obtained in the other crystals prepared without either or both of these two treatments. These ferromagnetic signals came from the spinel CuCr2X4, which was


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verified by their TC (e.g. the temperature dependence of magnetization in the CuCrS2 crystal without any treatment was shown in Fig. 4 (c)). As is clear in Fig. 4 (d), the crystal prepared with both the procedures does not show any magnetic transition of the spinel. The ferromagnetic signal from the spinel phase were discernible in the crystals grown without the quenching and with the high temperature treatment, which were apparently pure by XRD. These results evidently show that the magnetization measurements are more sensitive than XRD to detect the secondary spinel phase. In the temperature dependence of magnetization, even though the crystal contains a certain amount (~6%) of the ferromagnetic spinel impurity phase (the inset of Fig. 4(c)), the antiferromagnetic anomaly (the step-like drop) of CuCrS2 was clearly observed as similar to the pure case (the inset of Fig. 4(d)). This is partly the reason why all the previous studies could report the magnetic ground state of CuCrS2 as antiferromagnetic. Note, however in fact, that none of the previous work reported the field dependence of magnetization, which, as we demonstrated, is by far the sensitive probe of the magnetic impurity phase.

Figure 4. Field dependence of magnetization at 5 K of (a) CuCrS2 and (b) CuCrSe2 crystals prepared by each process. The magnetic field was applied parallel to the c-axis. Temperature


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dependence of the c-axis magnetization in a field cooling process at H = 100 Oe of CuCrS2 crystals prepared (c) without any treatment and (d) with both treatments.

From the measured M-H curves, we can quantify the amount of the spinel phase by comparison between the saturation magnetization (Ms) of the measured ferromagnetic signal and the reported saturation magnetization of spinels (Mspinel). Ms is estimated by extrapolating the linear magnetization in higher fields to H = 0. The impurity percentage is, then, given by (Ms/Mspinel) × 100, where Mspinel are 86.85 and 54.30 emu/g for CuCr2S4 [22] and CuCr2Se4 [23], respectively. The results are summarized in table I. The typical detection limit of XRD is about 1%, thus the magnetization detection is found to be actually more sensitive than XRD. The magnetic measurements unambiguously reveal that both of the two procedures, the waterquenching and the high temperature treatment, are indispensable to grow single crystals of CuCrX2 without the spinel secondary phases.

Table 1. Impurity percentage (amount of the spinel secondary phase) estimated by the magnetization measurements of crystals of CuCrS2 (left) and CuCrSe2 (right) grown from powders with/without the extra procedures.

0.9%

3.1%

~ 0%

without

with

without

5.5%

High-T treatment

7.0%

0.3%

with

with

Se Quenching

without

without

High-T treatment

with

S Quenching

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1.1%

~ 0%


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As the impurity percentage was quantitatively obtained, we can evaluate how the spinel impurity influences the transport properties. It is noted that there was no noticeable effect of S/Se-nonstoichiometry in ACrX2 even if we tried to produce intentional deficiencies. The temperature dependence of the in-plane resistivity of CuCrX2 crystals with different impurity concentrations is displayed in Fig. 5. The resistivity was dramatically increased with decreasing the amount of spinel impurity. (Note that resistivity in CuCrS2 is plotted on a logarithmic scale.) This may be attributed to the fact that spinel compounds have very low resistivity. In CuCrS2 crystals, in particular, by decreasing the impurity, the metallic conductivity was changed into the insulative behavior. This suggests that the variation in the reported conductivity of CuCrS2 was derived from the trace impurity of the spinel phase even if it was not detected by the XRD measurement. This result fully guaranteed that the pure CuCrS2 has both antiferromagnetic and insulating ground states, being able to host the multiferroic phenomena. On the other hand, the resistivity of CuCrSe2 crystals gradually increased with decreasing the amount of spinel impurity, but remained metallic. By decreasing the spinel impurity, resistivity anomaly was clearly observed around 50 K, which agreed with the antiferromagnetic transition in this compound [12].


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Figure 5. Temperature dependence of the in-plane resistivity of (a) CuCrS2 and (b) CuCrSe2 crystals with various amounts of the spinel impurity phase. Note that the vertical axis of (a) is a logarithmic scale.

Intrinsic Properties. As we established the quantitative magnetization detection of the impurity phase and the method to prepare pure single crystals, now the intrinsic properties of ACrX2 (A = Cu, Ag; X = S, Se) are discussed. Fig. 6 (a) shows in-plane resistivity of pure ACrX2 single crystals as a function of temperature plotted on a logarithmic scale. Resistivity for AgCrS2 below 150 K was too high to measure with our apparatus. The resistivity was systematically and dramatically changed by the combination of the A and the X compositions. When X = S, both Ag and Cu compounds ACrS2 were insulating, indicating that they offer excellent playgrounds for the multiferroic phenomena. On the other hand, low resistivity was observed in X = Se compounds. Changing A from Ag to Cu, the magnitude of resistivity became smaller in both cases of X = S and Se. As a result, CuCrSe2 was found to have the lowest resistivity. Around 80 K, AgCrSe2 gradually changed the temperature dependence from metallic to semiconducting, implying that it is a narrow band gap semiconductor.


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Figure 6. (a) Temperature dependence of the in-plane resistivity of ACrX2 crystals. (b) Anisotropy between the in-plane and the out-of-plane resistivity of ACrS2 crystals.

For sulfide compounds, we observed giant anisotropy between the in-plane and the out-ofplane resistivity (the latter is 2~3 orders of magnitude larger than the former as shown in Fig. 6 (b)). This behavior is consistent with the quasi two-dimensional crystal structure and the ionic conductivity of A+. Namely, the ionic conduction within the layers larger than that across the layers contributes to the gigantic anisotropy of resistivity in the insulative ACrS2. Moreover, the magnitude of resistivity has a positive correlation with the ionic radii of A+ (0.6 Å and 1.0 Å for Cu+ and Ag+). Similar large resistive anisotropy was also found in delafossite oxides, though the value of the anisotropy is usually lower than that of ACrS2, for instance, ρc/ρab ~ 35 for CuCrO2 [24] and ρc/ρab ~ 25 for CuAlO2 [25] at 300 K. These delafossites do not show ionic conductivity, while ACrS2 does. Those facts also support that the ionic conductivity plays an important role. Only using the ionic contribution framework, however, we cannot explain the dramatic change in resistivity induced by the X atom substitution. ACrSe2 have low resistivity (~ mΩcm), while usual solid ionic conductors do not have such low resistivity, indicating the major contribution is from electrons. In principle, the isovalent substitution of chalcogen X should not change the carrier concentration. The huge drop in resistivity without any carrier doping suggests that this system might have strong electron correlations and that ACrS2 is a Mott insulating state. Interestingly, AgCrS2 shows a tendency towards (anti)magnetic ordering (TN ~ 50 K) with an unconventional double-stripe structure [8]. The Se atom, on the other hand, has more extended porbital wave-functions than those of S, making orbital overlapping especially along the c-


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direction and destroying the Mott state. The observed effect of chalcogen substitution is similar to the one in NiS1-xSex, the prototypical Mott-Hubbard system [26]. The carrier densities of ACrX2 at 300 K were determined by the Hall measurements as shown in Fig. 7. The majority carriers for all the systems were found to be holes. AgCrS2 was unmeasurable due to high resistivity, while CuCrS2 has a low carrier density of 5.5 × 1017 cm-3. The values for AgCrSe2 and CuCrSe2 were 7.6 × 1019 and 1.3 × 1020 cm-3, respectively, which corresponded to heavily doped degenerated semiconductors. It is noted that the obtained carrier densities for selenium compounds are close to the ideal values (1018-19 cm-3) for the high performance thermoelectric materials [27]. In this regard, it is interesting to evaluate the thermoelectric properties of pure ACrSe2 and their derivatives.

Figure 7. Hall resistivity of ACrX2 single crystals at 300 K. Values in parentheses are the carrier concentration in units of cm-3.

CONCLUSIONS The pure single crystals of ACrX2 (A = Cu, Ag; X = S, Se) with plate or whisker shape were successfully grown by the modified chemical vapor transport technique, which employed two key heat-treatment processes for CuCrX2. Contamination of the spinel CuCr2S4 phase was


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confirmed to significantly modify the inherent nature of the samples. By avoiding the influence of the secondary phases of ferromagnetic metallic spinels, the intrinsic magnetic and transport properties were systematically revealed. Selenides were degenerated semiconductors with very low resistivity. On the other hand, sulfides were confirmed to hold the possibility as the multiferroic materials as they have highly insulating ground states. Furthermore, giant anisotropy between the ab-plane and the c-axis was found in resistivity of the sulfides. The observed anisotropy and drastic change in resistivity without any carrier doping were associated with the ionic conductivity and strong electron correlations. It is a very rare case that such many attractive structural and magnetic feathers, including the broken inversion symmetry, the strong electron correlations, and substantial ionic conductivity, coexisted in the same system. Furthermore, these compounds had high chemical stability in the air. By the success of preparing high quality single crystals, ACrX2 should become an interesting platform to investigate various phenomena.

ACKNOWLEDGMENT We would like to thank Dr. Y. Kohsaka for fruitful discussion. R.Y. was supported by a Research Fellowships for Young Scientists from Japan Society for the Promotion of Science (JSPS, Grant Number 15J11901).

REFERENCES [1] Takada, K.; Sakurai, H.; Muromachi, E. T.; Izumi, F.; Dilanian, R. A.; Sasaki, T. Nature 2003, 422, 53-55.


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Table of Contents Use Only Crystal Growth and Intrinsic Properties of ACrX2 (A = Cu, Ag; X = S, Se) without a Secondary Phase Rikizo Yano and Takao Sasagawa

By the CVT technique with CrCl3 and appropriate post heat-treatments, pure single crystals of (Cu,Ag)Cr(S,Se)2 were successfully grown. Their intrinsic magnetic and transport properties were systematically revealed by eliminating the influence of the secondary phases of ferromagnetic metallic spinels. The results suggest that ionic conductivity of (Cu,Ag) and strong electron correlations of Cr play important roles. [56 words]


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A = Cu, Ag; X = S, Se - American Chemical Society

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