Growth and characterization of millimeter-sized single crystals of CaFeAsF

The F-doped LnFeAsO (Ln = rare-earth elements), which has been abbreviated as the 1111 phase, is the first reported family with the highest critical transition temperature T_c in bulk in the Fe-based superconductors (FeSCs) [1]. However, investigations on the physical properties of the 1111-type FeSCs are restricted remarkably, compared with the 122 phase and 11 phase, due to the difficulties in obtaining sizable single crystals. As we know, it is essential to have high-quality single crystals when carrying out many experiments, including measurements of electrical transport, inelastic neutron diffraction, angle-resolved photoemission spectroscopy, and so on. Over the past several years, many efforts have been made to improve the quality and size of the single crystals. NaCl and KCl were first used as the flux and small single crystals with sizes of 20–70 μm were obtained [2]. Then more attempts, including the high-pressure method and the NaAs-flux method [3, 4], were made to further improve the growth processes. Up to now, the two goals, large size and high quality, have still not been achieved commendably. Recently, single crystals with a size of several millimeters were reported to be accessible in F-vacant and Na-doped CaFeAsF [5, 6], which is another type of 1111 phase without oxygen [7, 8], possibly due to the decrease of melting point in this fluorine-based system. As we know, a rather high T_c above 50 K can also be achieved by doping in this fluorine-based 1111 system [9–12]. More important information can be obtained owing to the availability of the sizable single crystals. To our knowledge, investigations on the single crystals of the parent phase CaFeAsF are still lacking.

Here we present the growth, structure, and transport measurements of the high-quality CaFeAsF single crystals with sizes of 1–2 mm. The single crystals were grown by the self-flux method. The structure details were obtained from the refinement of the single-crystal x-ray diffraction data. The structural transition at 121 K was confirmed by the resistivity, magnetoresistance, and magnetic susceptibility measurements. A feature coming from the antiferromagnetic transition was also observed in the resistivity data.

High-quality CaFeAsF single crystals were grown using the self-flux method with CaAs as the flux. First, the starting materials Ca granules (purity 99.5%, Alfa Aesar) and As grains (purity 99.995%, Alfa Aesar) were mixed in 1:1 ratio. Then the mixture was sealed in an evacuated quartz tube and followed by a heating process at 700 °C for 10 h to get the CaAs precursor. CaAs, FeF2 powder (purity 99%, Alfa Aesar) and Fe powder (purity 99+%, Alfa Aesar) were mixed together in the stoichiometric ratio 10:1:1, and the mixture was placed in an alumina crucible. Finally, the crucible was sealed in a quartz tube with a vacuum. All the weighing and mixing procedures were carried out in a glove box with a protective argon atmosphere. The quartz tube was heated at 950 °C for 40 h firstly, and then it was heated up to 1230 °C where it remained for 20 h. Finally it was cooled down to 900 °C at a rate of 2 °C h⁻¹ followed by a quick cooling down to room temperature.

The microstructure was examined by scanning electron microscopy (SEM, Zeiss Supra55). The composition of the single crystals was checked and determined by energy dispersive x-ray spectroscopy (EDS) measurements on an Oxford Instruments device. The crystals were first checked using a DX-2700 type powder x-ray diffractometer. The detailed structure was characterized and analyzed by single-crystal x-ray diffraction measurements on a Bruker D8 Quest diffractometer equipped with the graphite-monochromatized Mo ${K}_{\alpha }$ radiation. The magnetic susceptibility measurement was carried out on the magnetic property measurement system (Quantum Design, MPMS 3). The electrical resistivity and magnetoresistance (MR) were measured using a four-probe technique on the physical property measurement system (Quantum Design, PPMS) with magnetic field up to 9 T. For the MR measurements, the magnetic field was oriented parallel to the c-axis of the samples and the data were measured for both positive and negative field orientations to eliminate the effect of the Hall signals.

The typical dimension of the single crystals is 1.2 × 1.0 × 0.1 mm³. The morphology was examined by the scanning electron microscopy. An SEM picture for the CaFeAsF single crystal can be seen in figure 1(a), which shows a flat surface and some terrace-like features. An enlarged view of this picture can be seen in figure 1(b). The composition of the crystals was characterized by energy-dispersive x-ray spectroscopy (EDS) measurements. We measured the EDS at different positions of the sample. Here we show a typical result in figure 1(c) and table 1, which revealed that the ratio of Ca:Fe:As is close to the stoichiometric ratio. The content of the light element F is difficult to determine precisely based on EDS measurements. The structure of the crystals was first check by a powder x-ray diffractometer, where the x-ray was incident on the ab-plane of the crystal. The diffraction pattern is shown in figure 2. All the diffraction peaks can be indexed to the tetragonal ZrCuSiAs-type structure (see the inset of figure 2). Only sharp peaks along the (00 l) orientation can be observed, suggesting a high c-axis orientation. The full width at half maximum (FWHM) of the diffraction peaks is only about 0.10° after deducting the ${K}_{\alpha 2}$ contribution, indicating a rather fine crystalline quality. The c-axis lattice constant was obtained to be 8.584 Å by analyzing the diffraction data.

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We used high-resolution single-crystal x-ray diffraction to study the structural details of our sample. The diffraction data were collected at room temperature by the ω- and φ-scan method. The crystal structure was solved by SHELXS-2014 and refined by SHELX-2014 [13]. The parameters for the data collection and structure refinement are listed in table 2. The values of R${}_{1}$ and ${{wR}}_{2}$ are much smaller than the previously reported polycrystalline results [10], and also small compared to the Na-doped single crystalline system [6], indicating the high-quality of our sample and the reliability of our refinements. As shown in table 3, the final cell constants are determined to be a = b = 3.8774(4) Å, c = 8.5855(10) Å. It is clear that the c-axis lattice constants are very close to that obtained from the data in figure 2. In addition, the a- and c-axis lattice constants determined from our experiment are consistent with the polycrystalline samples reported previously [9, 10]. Compared to the Na-doped single crystalline samples, the a-axis lattice constant is similar while the c-axis constant is clearly smaller [6]. The anion (As) height relative to Fe layer is a bit larger than the optimal value (1.38 Å) for the highest ${T}_{{\rm{c}}}$ in FeSCs [14]. The atomic coordinates from the refinement are shown in table 4, which also confirms the structure obtained earlier on the basis of powder data, with a difference of about 0.16% for the c-axis position of the As element [10]. The bond angles ${\delta }_{\mathrm{As}-\mathrm{Fe}-\mathrm{As}}$ deviate a bit from the optimal value of about 109.47°.

Table 2.  Parameters for the data collection and structure refinement of CaFeAsF.

Theta range for data collection	4.749 to 27.508°
Index ranges	-4 $\leqslant$h $\leqslant 5$
	-5 $\leqslant$k $\leqslant 5$
	-11 $\leqslant$l $\leqslant 11$
Reflections collected	2035
Refinement method	Full-matrix least-squares on F2
Refinement program	SHELXL-2014 (Sheldrick, 2014)
Data /restraints /parameters	114 /0 /12
Goodness-of-fit on F2	1.224
Final R indices	R1 = 0.0139
	wR2 = 0.0318
Weighting scheme	w = 1/[${\sigma }^{2}$(${{\rm{F}}}_{o}^{2}$)+0.6368P]
	where P = (${F}_{o}^{2}+2{F}_{c}^{2}$)/3
Extinction coefficient	0.014(3)
Largest diff. peak and hole	0.655 and -0.364 eÅ${}^{-3}$
R.M.S. deviation from mean	0.122 eÅ${}^{-3}$

Table 3.  Refined lattice constants for the CaFeAsF single crystal.

Chemical formula	CaFeAsF
Formula weight	189.85 g mol−1
Temperature	296(2) K
Wavelength	0.71073 Å
Crystal system	tetragonal
Space group	P4/nmm (No. 129)
Z	2
Unit cell dimensions	a = 3.8774(4) Å, α = 90°
	b = 3.8774(4) Å, β = 90°
	c = 8.5855(10) Å, γ = 90°
Volume	129.076(4) Å${}^{3}$
Bond angle (${\delta }_{\mathrm{As}-\mathrm{Fe}-\mathrm{As}}$)	107.82(6)° × 2
	110.30(4)° × 4
Anion height	hAs = 1.413 Å
Density (calculated)	4.885 g/cm3
Absorption coefficient	20.223 mm−1
F(000)	176

The resistivity, MR, and magnetic susceptibility change the variation tendency at the same temperature, 121 K, on the temperature-dependent curves, as revealed in figures 3(a)–(c). This seems to be a common feature in most of the parent phase of FeSCs associated with the structural and the spin-density-wave (SDW)-type antiferromagnetic transition. This transition temperature is a little higher than the polycrystalline results (118–120 K) [9, 10]. Temperature dependence of resistivity is shown in figure 3(a). Above 121 K, the resistivity increases almost linearly with the decrease of temperature. We note that different reports based on polycrystalline samples are rather conflicted [7–10]. Only one result reported on SrFeAsF by Tegel et al shows a similar tendency, compared with our data [8]. We argue that the data from single-crystal samples reveals the intrinsic properties since the scattering processes are not affected by the grain boundaries. Moreover, the transition at 121 K is sharper than the results from polycrystalline samples. Below that temperature, the resistivity decreases with cooling and a clear kink can be observed at about 110 K, as indicated by the green dashed line. These two characteristic temperatures with the interval of 11 K are reminiscent of the reported ${T}_{\mathrm{str}}$ = 150 K and ${T}_{N}$ = 138 K in the another 1111 phase LnFeAsO (Ln = La, Ce) [15, 16], where ${T}_{\mathrm{str}}$ and ${T}_{N}$ are the transition temperatures from tetragonal to orthogonal structure and that from paramagnetic to SDW-type antiferromagnetic phase, respectively. We note that such two distinct transition temperatures have also been detected from the resistivity data in the Co-doped BaFe${}_{2}{{\rm{As}}}_{2}$ system [17]. So it is very likely that these two transition temperatures are ${T}_{\mathrm{str}}$ = 121 K and ${T}_{N}$ = 110 K for the present CaFeAsF system.

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In is paper, MR is expressed as $\Delta \rho /{\rho }_{0}=[\rho (9{\rm{T}})-{\rho }_{0}]/{\rho }_{0}$, where ρ$(9\;T)$ and ${\rho }_{0}$ are the resistivity under the field 9 T and zero field, respectively. In figure 3(b), we show the temperature dependence of MR. The magnitude of MR decreases with the increase of the temperature monotonously until ${T}_{\mathrm{str}}$ and vanishes above this temperature. These observations suggest that the MR in this system is associated with the magnetic and electronic structures, which are affected by the structure and the SDW-type antiferromagnetic transitions remarkably. The transition on the M–T curve shows the feature of an antiferromagnetic transition, as shown in figure 3(c). In the high-temperature non-magnetic normal state, a linear-temperature-dependent behavior can be observed. This is a non-Curie–Weiss-like paramagnetic behavior and cannot be understood within a simple mean-field picture. This behavior should be very important to understand the mechanism of high-${T}_{{\rm{c}}}$ superconductivity because it was also observed in undoped and highly underdoped cuprates [18]. In the pnictide compounds, this feature was interpreted by the antiferromagnetic fluctuations with the local SDW correlations [19].

In summary, high-quality and sizable single crystals of CaFeAsF were grown successfully by the self-flux method. The single-crystal x-ray diffraction measurements were carried out and the structure details were refined based on the data. The resistivity, magnetoresistance, and magnetic susceptibility show clear different behaviors below and above 121 K. The critical temperatures of the structure and antiferromagnetic transition were determined to be ${T}_{\mathrm{str}}$ = 121 K and ${T}_{N}$ = 110 K, respectively. Our results supply a platform to study the intrinsic properties of the 1111 phase of FeSCs.

This work is supported by the National Natural Science Foundation of China (No. 11204338), the 'Strategic Priority Research Program (B)' of the Chinese Academy of Sciences (no. XDB04040300, XDB04040200 and XDB04030000) and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (no. 2015187).