Single Crystal Growth and X-ray Diffraction Characterization of a Quasi-Spin Chain Compound, Li₂CuO₂

Abstract

We report the growth of single crystals and X-ray diffraction characterization of the quasi-one-dimensional spin chain compound, Li₂CuO₂. 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 $\mathit{b}$-axis. The as-grown crystals were characterized using powder X-ray diffraction and Laue back reflection. The I-centered orthorhombic, $\mathit{Immm}$, structure was confirmed. Crystal structure parameters were determined to be $\mathit{a}$ = 3.6744 Å, $\mathit{b}$ = 2.8600 Å, and $\mathit{c}$ = 9.4257 Å from Rietveld analysis. Our work helps to remove obstacles to the synthesis and study of a model cuprate system, Li₂CuO₂, facilitating the use of experimental probes that require sizable crystals.

1. Introduction

Intense research focus in low-dimensional materials centers around one-dimensional (1D) spin chains (e.g., Sr₂CuO₃, CuGeO₃, SrCuO₂) and spin ladders (e.g., Sr₁₄Cu₂₄O₄₁, SrCu₂O₃) [1,2,3], owing to a myriad of unusual low-temperature properties governed by quantum effects. Refs. [2,3,4] These properties include various types of magnetic ground states for half-integer [5,6,7] and integer spin systems (e.g., the Haldane chain) [8,9], as well as the separation of spin (spinon) and charge (holon) degrees of freedom in Sr₂CuO₃ [10,11,12,13], spin-Peierls transitions in CeCuGe₃ [14], etc. These phenomena are entirely governed by quantum-mechanical behavior and lack three-dimensional analogs. As observed, compounds containing, $S=1/2$, Cu²⁺, ions offer an excellent experimental platform for realizing such networks [1,2,3,5,6,7,10,11,12,13,14] because of the enhanced quantum fluctuations resulting from Cu²⁺ ions and reduced dimensionality.

Recently, there has been considerable interest in the quasi 1D compound, Li₂CuO₂. Refs. [15,16,17,18,19,20,21,22] Li₂CuO₂ crystallizes into an orthorhombic structure ($\mathit{Immm}$, space group $\#71$) with $\mathit{a}$ = 3.6544 Å, $\mathit{b}$ = 2.8602 Å, and $\mathit{c}$ = 9.3774 Å at ambient pressure. Refs. [23,24] As depicted in Figure 1a, the most notable characteristic of Li₂CuO₂ is the formation of quasi-one-dimensional linear chains composed of CuO₂ along the $\mathit{b}$-axis, separated by Li-O groups along the $\mathit{c}$-axis. Refs. [24,25] Due to the simple CuO₄ square planar coordination along the $\mathit{b}$-axis (see Figure 1b), these spin chains run parallel to each other with a Cu-O-Cu bond angle of approximately 94°. Refs. [24,26] The copper atoms within these structural chains exhibit ferromagnetic (FM) coupling, resulting in ferromagnetic alignment along the $\mathit{b}$-axis, which are antiferromagnetically (AFM) coupled between the neighboring chains in the $\mathit{c}$-axis. Refs. [17,24,26] The remarkable interplay between one and three-dimensional magnetic interactions distinguishes Li₂CuO₂ from other related compounds, as evidenced by, for instance, the presence of long-range AFM order observed below $T_N$ ≈ 9 K with a Curie-Weiss temperature, ${\Theta }_{cw}$ = −42 K, ref. [24] a field-induced weak ferromagnetic component below $T_2$ > 2.6 K, refs. [27,28] and relatively large magnetic moments of O²⁻ ions in Li₂CuO₂ of about 0.2 $\mathsf{\mu }$_B (largest for any low-dimensional cuprate system), and Zhang-Rice type excitations. Refs. [29,30] Despite the tremendous theoretical [15,19,20,27] and experimental studies, refs. [15,17,24,26,27] the precise nature of the magnetic interactions in this charge transfer insulator [31,32,33] has remained a topic of contentious debate.

It is noteworthy that most studies have focused on polycrystalline, Li₂CuO₂. 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 Li₂CuO_2−δ, where $\delta =0-0.3$, within their structure. Refs. [16,18] Consequently, the magnetic susceptibility, $\chi \left(T\right)$, data at low temperatures for these single crystals of Li₂CuO₂ 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 Li₂CuO₂ using the more accessible high-temperature solution growth technique, also known as the flux method.

Here, we present a successful single crystal growth of Li₂CuO₂. 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 $\mathit{b}$-axis. We demonstrate the high quality of our single crystals through X-ray diffraction characterization. To the best of our knowledge, we present the first instance of Li₂CuO₂ crystal growth via flux growth.

2. Materials and Methods

Single crystals of Li₂CuO₂ were grown using Li₂CO₃ flux. The precisely weighed starting materials, Li₂CO₃ (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 mm³.

We studied the crystal structure and the purity of the single crystal Li₂CuO₂ using powder X-ray diffraction (PXRD). The PXRD patterns were measured on pulverized Li₂CuO₂ 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].

3. Results and Discussions

3.1. 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 Li₂CuO₂ 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 $\mathit{b}$-axis (see Laue X-ray diffraction below).

3.2. Structural Characterizations

To determine the crystal structure of the as-grown Li₂CuO₂ 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 2$\theta$ 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 $\mathit{Immm}$ group in Ref. [24] and our analysis below.

The PXRD pattern of the pulverized Li₂CuO₂ 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 Li₂CuO₂ phase. We determined the lattice parameters of Li₂CuO₂ to be $\mathit{a}$ = 3.67442 Å, $\mathit{b}$ = 2.86001 Å, $\mathit{c}$ = 9.42566 Å with $\alpha$ = $\beta$ = $\gamma$ = 90°, and we obtained the goodness-of-fit parameters ${\chi }^2$ = 3.785 and R_wp = 0.584. Our Rietveld result is consistent with the orthorhombic $\mathit{Immm}$. The atomic coordinates and other parameters considered for the measurements, as well as detailed refined parameters for Li₂CuO₂, are presented in

Table 1. Structural parameters obtained by refining X-ray powder diffraction for Li₂CuO₂ at room temperature, shown in Figure 3b with a space group $\mathit{Immm}$ ($\#71$). The I-centered orthorhombic lattice constants are $\mathit{a}$ = 3.6744 Å, $\mathit{b}$ = 2.8600 Å, $\mathit{c}$ = 9.4257 Å, and $\alpha$ = $\beta$ = $\gamma$ = 90°, with goodness of fit parameters, ${\chi }^2$ = 3.785, and R_wp = 0.584.

Atom	Wyck	x	y	z	$\mathit{Occ}.$	B (Å)
Li	4j	0.5	0	0.18487	1	0.00032
Cu	2b	0	0.5	0.5	1	0.00032
O	4i	0	0	0.36189	1	0.00032

. These values agree well with previously reported values [24,34] as shown in

Table 2. Comparison of the experimental and computed lattice constants, $\mathit{a}$, $\mathit{b}$, and $\mathit{c}$ for Li₂CuO₂ in the current study and literature data.

	a (Å)	b (Å)	c (Å)	Cu-O-Cu (°)	Cu-Cu (Å)	Cu-O (Å)	Reference
Experiment	3.6744	2.8600	9.4257	95.37	2.8600	1.9338	this work
Experiment	3.6615	2.8627	9.3925	93.96	2.8628	1.9577	Ref. [24]
Experiment	3.65	2.86	9.38	-	-	-	Ref. [34]
Experiment	3.6614	2.8648	9.3969	91		1.987	Ref. [16]
Experiment: Li2CuO1.71	3.6609	2.8638	9.3934	95.55	-	-	Ref. [16]
DFT, GGA + PBE	3.5779	2.8628	9.3926	-	-	1.9162	Ref. [21]

. We found a small amount of impurity (less than 4%) in the PXRD pattern. It is determined to be the flux component Li₂CO₃ (ICSD: 66941), appearing at 2$\theta$ = 21.1976° and 31.5°, indicated by * (dark grey) in Figure 3b.

3.3. 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 Li₂CuO₂ 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).

4. Conclusions

We present a comprehensive report on the single crystal growth using the flux method and the structural characterization of Li₂CuO₂ single crystals. Through a detailed structural refinement of the PXRD diffraction data, we determine that the material crystallizes in a $\mathit{Immm}$ ($\#71$) space group, with orthorhombic symmetry, and assess the lattice parameters of Li₂CuO₂. Our observations indicate that the as-grown samples of Li₂CuO₂ measure several millimeters along the crystallographic $\mathit{b}$-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, Li₂CuO₂, facilitating the use of experimental probes that require sizable crystals.