Single Crystal Growth and X-ray Diffraction Characterization of a Quasi-Spin Chain Compound, Li 2 CuO 2

: We report the growth of single crystals and X-ray diffraction characterization of the quasi-one-dimensional spin chain compound, Li 2 CuO 2 . The single crystals were grown using the high-temperature solution growth technique. The resulting blade-shaped crystals exhibit a shiny black color, with dimensions reaching several millimeters along the crystallographic b -axis. The as-grown crystals were characterized using powder X-ray diffraction and Laue back reflection. The I-centered orthorhombic, Immm , structure was confirmed. Crystal structure parameters were determined to be a = 3.6744 Å, b = 2.8600 Å, and c = 9.4257 Å from Rietveld analysis. Our work helps to remove obstacles to the synthesis and study of a model cuprate system, Li 2 CuO 2 , facilitating the use of experimental probes that require sizable crystals.

Refs. [23,24,27,34,35] Bulk characterizations were done on single crystals grown by the floating-zone (FZ) technique.Refs.[16,18,26,28,30,33,36] These single crystals were grown under partial oxygen pressure and exhibited some oxygen vacancies.This oxygen partial pressure can strongly influence the Lithium self-diffusion, leading to the formation of Li 2 CuO 2−δ , where δ = 0 − 0.3, within their structure.Refs.[16,18] Consequently, the magnetic susceptibility, χ(T), data at low temperatures for these single crystals of Li 2 CuO 2 exhibit a slightly more pronounced Curie-like increase, which varies between samples, suggesting possible sample contamination related to surface sensitivity to moisture.Refs.[16][17][18] This variation may stem from differences in the thickness of inclusions of the second phase or micro-cracks at cleaved planes.Ref. [18] Given that advanced FZ techniques are not widely available in most scientific laboratories, we were inspired to investigate the synthesis of Li 2 CuO 2 using the more accessible high-temperature solution growth technique, also known as the flux method.
Here, we present a successful single crystal growth of Li 2 CuO 2 .We utilized a conventional flux method and synthesized blade-shaped single crystals which are shiny black in color and air-sensitive [18], with dimensions reaching up to several millimeters along the crystallographic b-axis.We demonstrate the high quality of our single crystals through Xray diffraction characterization.To the best of our knowledge, we present the first instance of Li 2 CuO 2 crystal growth via flux growth.

Materials and Methods
Single crystals of Li 2 CuO 2 were grown using Li 2 CO 3 flux.The precisely weighed starting materials, Li 2 CO 3 (99.999%,Alfa Aesar, Haverhill, MA, USA), and CuO (99.995%,Alfa Aesar), were homogeneously mixed in a 4:1 ratio.This mixture was carefully transferred into an alumina crucible, covered with a lid, and gradually heated to 900 • C over 5 h, and held at 900 • C for 72 h (3 days) in air.Finally, the furnace was slowly cooled to room temperature for over 30 h.We obtained blade-shaped shiny single crystals (shown in Figure 2) with typical dimensions of 3 × 0.3 × 0.1 mm 3 .We studied the crystal structure and the purity of the single crystal Li 2 CuO 2 using powder X-ray diffraction (PXRD).The PXRD patterns were measured on pulverized Li 2 CuO 2 single crystals at room temperature using a Proto AXRD Benchtop powder diffractometer.We utilized Cu-K α X-ray radiation, and the structure was quantitatively analyzed by the Rietveld method using the GSAS program package [37].
We identified the crystallinity and crystallographic orientations of our single crystals using a Proto Laue-COS system, utilizing the Laue back reflection method.We used the Cu X-ray tube for X-ray generation with 40 kV and 30 mA.Due to the small width of the blade, we used a 0.8 mm pinhole aperture.We mount the single crystal with one of the flat surfaces perpendicular to the incident X-rays and the long blade direction vertically.Approximately 16 scans were performed over an extended duration (8 min) and averaged for clear diffraction patterns.The Laue patterns were analyzed by the Cologne Laue Indexation Program (CLIP) [38].

Single Crystal Growth
Figure 2a displays optical images of as-grown single crystals in the flux matrix as well as two separate batches of pulverized Li 2 CuO 2 single crystals on a millimeter-sized grid (see Figure 2b,c).This demonstrates that our synthesis conditions are highly reproducible.In the flux matrix, we obtained black, shiny blade-shaped single crystals with dimensions typically ranging from 2-3 mm in length, 0.3 mm in width, and 0.1 mm in thickness, with the longest dimension aligned along the crystallographic b-axis (see Laue X-ray diffraction below).

Structural Characterizations
To determine the crystal structure of the as-grown Li 2 CuO 2 single crystals, we conducted single crystal diffraction and PXRD measurements in the Bragg-Brentano geometry at room temperature.Figure 3a displays the single crystal diffraction pattern.We observed peaks corresponding to the (1, 0, 1) and (2, 0, 2) planes at approximately 2θ values of 25.96 • and 53.48 • , respectively.Earlier studies done on the single crystalline samples (using FZ technique) by Chung et al. [22] showed the resulting crystalline boules were faceted and exhibited a tendency to cleave along (1, 0, 1) planes similar to our single crystals [33].It is consistent with an orthorhombic structure within the Immm group in Ref. [24] and our analysis below.The PXRD pattern of the pulverized Li 2 CuO 2 single crystals from the crystal batch in Figure 2c are displayed in Figure 3b.We performed detailed structural refinement using the structure model described in ref. [24].The solid (red) curves overlaid on the data represent the Rietveld refinement in Figure 3b.We found that the PXRD pattern predominantly consists of the correct Li 2 CuO 2 phase.We determined the lattice parameters of Li 2 CuO 2 to be a = 3.67442 Å, b = 2.86001 Å, c = 9.42566 Å with α = β = γ = 90 • , and we obtained the goodness-of-fit parameters χ 2 = 3.785 and R wp = 0.584.Our Rietveld result is consistent with the orthorhombic Immm.The atomic coordinates and other parameters considered for the measurements, as well as detailed refined parameters for Li 2 CuO 2 , are presented in Table 1.These values agree well with previously reported values [24,34] as shown in Table 2.We found a small amount of impurity (less than 4%) in the PXRD pattern.It is determined to be the flux component Li 2 CO 3 (ICSD: 66941), appearing at 2θ = 21.1976• and 31.5 • , indicated by * (dark grey) in Figure 3b.3b with a space group Immm (#71).The I-centered orthorhombic lattice constants are a = 3.6744 Å, b = 2.8600 Å, c = 9.4257 Å, and α = β = γ = 90 • , with goodness of fit parameters, χ 2 = 3.785, and R wp = 0.584.

Laue X-ray Diffraction
We examined the crystallinity of our crystal selected from the batch shown in Figure 2b, using the Laue back reflection method across the as-grown crystal facet.The resulting Laue image from a Li 2 CuO 2 single crystal is shown in Figure 4a and simulated Laue patterns together with the data are presented in Figure 4b.Our Laue pattern shows no visible impurity phases or misaligned crystals, indicating a high quality of the single crystal.We find that the surface perpendicular to the incident X-rays is (1, 0, 1) plane, and the horizontal direction is the high symmetry direction to (1, 0, 0).We also find that the vertical direction is to [0, 1, 0], which is along the long blade direction.Considering the proposed quasi-one-dimensional chains in this compound, it is natural that the crystal extends along the chain direction (the crystallographic b-axis).

Conclusions
We present a comprehensive report on the single crystal growth using the flux method and the structural characterization of Li 2 CuO 2 single crystals.Through a detailed structural refinement of the PXRD diffraction data, we determine that the material crystallizes in a Immm (#71) space group, with orthorhombic symmetry, and assess the lattice parameters of Li 2 CuO 2 .Our observations indicate that the as-grown samples of Li 2 CuO 2 measure several millimeters along the crystallographic b-axis.Further, Laue back reflection X-ray measurements indicate the high quality of the single crystal.Our work helps to remove obstacles to the synthesis and study of a model cuprate system, Li 2 CuO 2 , facilitating the use of experimental probes that require sizable crystals.

Figure 1 .
Figure 1.(a) Schematics of a unit cell of Li 2 CuO 2 .The magnetic moments within the CuO 2 planes along b-axis are FM ordered, whereas neighboring planes are coupled AFM along the c-axis.Red full circles are the oxygen atoms, green color circles are the Cu atoms and the blue circles are the Li atoms.Solid bicolor lines are the nearest neighbor Cu-O and Li-O bonds.(b) Schematics of a two unit cell of Li 2 CuO 2 in the bc plane.

Figure 2 .
Figure 2. (a) As-grown single crystals of Li 2 CuO 2 in the flux matrix.(b,c) Single crystals pulverized from the flux matrix of Li 2 CuO 2 from two different batches on a millimeter-sized grid.

Figure 3 .
Figure 3. (Color online) (a) Single crystal X-ray diffraction pattern of Li 2 CuO 2 .(b) Powder X-ray diffraction pattern (black) of pulverized single-crystals of Li 2 CuO 2 .The solid (red) curves overlaying the data represent the fit from Rietveld refinement.The blue and grey solid lines represent the reference peaks of Li 2 CuO 2 and Li 2 CO 3 , respectively.The * symbols show the impurity peaks of a flux component, Li 2 CO 3 .

Table 1 .
Structural parameters obtained by refining X-ray powder diffraction for Li 2 CuO 2 at room temperature, shown in Figure

Table 2 .
Comparison of the experimental and computed lattice constants, a, b, and c for Li 2 CuO 2 in the current study and literature data.