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Stoichiometry and Phase Control in K$_{1-x}$CrSe$_2$ via Self-Flux Synthesis

Felix Eder, Catherine Witteveen, Enrico Giannini, Fabian O. von Rohr

TL;DR

This work shows that self-flux synthesis with a fixed nominal composition can reproducibly stabilize three distinct K1-xCrSe2 phases by simply varying the quenching temperature, linking stoichiometry to crystal structure and phase purity. Single-crystal X-ray diffraction reveals three related but structurally distinct phases, including the first refinements for K0.68(2)CrSe2 and K0.65(3)CrSe2, with clear changes in in-plane and interlayer parameters driven by potassium vacancies. Magnetic measurements on stoichiometric KCrSe2 uncover a low-field spin-flop, a Néel temperature of $T_N = 86$ K, and a near-degeneracy between AFM and FM states, as evidenced by a small saturation field and field-sensitive $T_N$. Overall, the study demonstrates that a simple synthesis parameter—the quenching temperature—acts as a robust handle to navigate the phase diagram and tune the magnetic properties of delafossite-type materials, positioning flux synthesis as a strategic tool for phase control and discovery in layered magnets.

Abstract

Layered delafossite-type magnetic materials, such as KCrSe$_2$, are promising platforms for studying magnetic systems and potential frustration on triangular lattices. Synthesis, structure-type control, and off-stoichiometries remain major challenges in the investigation of these delafossite-type magnets. Starting from the same self-flux composition (K:Cr:Se = 8:1:8), we isolated three distinct K$_{1-x}$CrSe$_2$ phases with $x$ = 0, 0.13--0.17, and 0.32--0.35, each adopting a different structure type depending on the quenching temperature applied. The phase evolution indicates a sequence of transformations during synthesis between compounds with varying degrees of potassium deficiency. Building on these insights into phase stability and crystal growth, we successfully grew single crystals of full-stoichiometric KCrSe$_2$ -- enabling direction-dependent magnetization measurements. These measurements reveal a pronounced field dependence of the Néel temperature at low external fields, as well as a weak metamagnetic transition. Our findings demonstrate that even a simple parameter -- such as quenching temperature -- can be used to control stoichiometry, direct phase formation, and ultimately tune the magnetic properties of delafossite-type materials.

Stoichiometry and Phase Control in K$_{1-x}$CrSe$_2$ via Self-Flux Synthesis

TL;DR

This work shows that self-flux synthesis with a fixed nominal composition can reproducibly stabilize three distinct K1-xCrSe2 phases by simply varying the quenching temperature, linking stoichiometry to crystal structure and phase purity. Single-crystal X-ray diffraction reveals three related but structurally distinct phases, including the first refinements for K0.68(2)CrSe2 and K0.65(3)CrSe2, with clear changes in in-plane and interlayer parameters driven by potassium vacancies. Magnetic measurements on stoichiometric KCrSe2 uncover a low-field spin-flop, a Néel temperature of K, and a near-degeneracy between AFM and FM states, as evidenced by a small saturation field and field-sensitive . Overall, the study demonstrates that a simple synthesis parameter—the quenching temperature—acts as a robust handle to navigate the phase diagram and tune the magnetic properties of delafossite-type materials, positioning flux synthesis as a strategic tool for phase control and discovery in layered magnets.

Abstract

Layered delafossite-type magnetic materials, such as KCrSe, are promising platforms for studying magnetic systems and potential frustration on triangular lattices. Synthesis, structure-type control, and off-stoichiometries remain major challenges in the investigation of these delafossite-type magnets. Starting from the same self-flux composition (K:Cr:Se = 8:1:8), we isolated three distinct KCrSe phases with = 0, 0.13--0.17, and 0.32--0.35, each adopting a different structure type depending on the quenching temperature applied. The phase evolution indicates a sequence of transformations during synthesis between compounds with varying degrees of potassium deficiency. Building on these insights into phase stability and crystal growth, we successfully grew single crystals of full-stoichiometric KCrSe -- enabling direction-dependent magnetization measurements. These measurements reveal a pronounced field dependence of the Néel temperature at low external fields, as well as a weak metamagnetic transition. Our findings demonstrate that even a simple parameter -- such as quenching temperature -- can be used to control stoichiometry, direct phase formation, and ultimately tune the magnetic properties of delafossite-type materials.

Paper Structure

This paper contains 8 sections, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimental
  3. Results and discussion
  4. Synthesis and SXRD
  5. Phase purity
  6. Influence of annealing time on the formation of KCrSe2
  7. Direction-dependent magnetic properties of KCrSe2 single crystals
  8. Conclusion

Figures (4)

  • Figure 1: Phase and Stoichiometry Control: (a) Scheme of self-flux synthesis consisting of assembly of a Canfield crucible set, sealing the setup in a quartz ampoule, heating up to achieve a homogenous melt, slow cooling to grow large crystals, and quenching during hot-centrifugation. (b) The temperature profile applied for synthesis of various K_1--xCrSe2 phases. (c) The crystal structures of full stoichiometric KCrSe2, incommensurately modulated K_0.866CrSe2 and K_0.68(2)CrSe2 viewed along b. The different coordinations of K cations by Se atoms of the CrSe2 layers are highlighted at the bottom.
  • Figure 2: PXRD patterns acquired from samples quenched at (a) 850 °C (b) 750 °C (reproduced from [Eder2025]. Copyright 2025 American Chemical Society.), and (c) 600 °C. Background was subtracted after Rietveld refinement for clarity. In all cases, a minor impurity from residual K/Se flux is visible. The three patterns differ markedly in both peak positions and intensities, enabling clear distinction between the resulting phases. These differences reflect changes in symmetry and lattice parameters associated with varying potassium content.
  • Figure 3: The transformation of K_0.83--0.87CrSe2 to KCrSe2 (a) Selected areas of PXRD patterns of samples quenched from T = 600 °C after t = three weeks (top), T = 600 °C, t = 1 d (mid), and T = 750 °C (bottom). The 2$\theta$ ranges on the x-axis have been chosen to highlight the multi-phase pattern in the middle. The complete diffraction patterns are displayed in the SI. Positions of KCrSe2, K_0.87CrSe2 main and K_0.87CrSe2 satellite reflections are marked in orange, dark blue and blue, respectively. The background was subtracted for clarity. (b) SEM-SE picture taken at 20 kV acceleration voltage of sample quenched after less than one day at 600 °C. Recrystallizing shards of KCrSe2 are visible with a background of incompletely removed K2Se2 flux. EDS maps of the constituting elements are displayed at the bottom.
  • Figure 4: Magnetic properties of oriented KCrSe2 single crystals. (a) Field-dependent magnetic moment at 2 K. The inset highlights the metamagnetic transition at very low fields for H$\parallel$ab. Data were corrected for demagnetization effects. (b) Field-dependent magnetic moment at various temperatures for H$\parallel$ab. (c) Temperature-dependent molar magnetic susceptibility at various external fields. (d) Field-dependence of the Néel temperature for H$\parallel$ab.