Formation of single-phase disordered CsxFe2−ySe2

Abstract

A single-phase high pressure (HP) modification of CsxFe2-ySe2 was synthesized at 11.8 GPa at ambient temperature. Structurally this polymorph is similar to the minor low pressure (LP) superconducting phase, namely they both crystallize in a ThCr2Si2-type structure without ordering of the Fe vacancies within the Fe-deficient FeSe4 layers. The HP CsxFe2-ySe2 polymorph is found to be less crystalline and nearly twice as soft compared to the parent major and minor phases of CsxFe2-ySe2. It can be quenched to low pressures and is stable at least on the scale of weeks. At ambient pressure the HP polymorph of CsxFe2-ySe2 is expected to exhibit different superconducting properties compared to its LP minor phase (Tc = 27 K). IntroductionThe phenomena of superconductivity was discovered more than a century ago (Hg with a Tc = 4 K) [1], but to date no material exhibiting a superconducting response at ambient temperature has been reported. While the recently discovered FeSe phase also possesses a low critical temperature (Tc = 8 K [2]), there exist a number of ways to enhance its superconducting performance. For instance, the transition temperature drastically increases for FeSe in a monolayer form (Tc > 100 K [3]). In addition, its Tc can be enhanced by intercalation: for the Fe-deficient FeSe systems intercalated by alkali metals (A), the Tc was reported to reach 30 K [4][5][6]. Further increase in the Tc of AxFe2-ySe2 can be achieved through a control of Fe occupancies. Phases with Tc exceeding 40 K can be achieved through specific synthetic procedures, including precise control of stoichiometry and annealing conditions, and are characterized by fully, or close to fully, occupied Fe sites [7][8][9]. In addition, their formation can be mediated by NH3 molecules [10][11][12][13].Application of external pressure is yet another tool that allows control of physical properties, including FeSe-based superconducting materials. For the FeSe phase itself, high-pressure (HP) induces more than a four-fold increase in its critical temperature (Tc = 36 K around 7 GPa [14]). Above 7 GPa, structural transformation of FeSe into a topologically-different polymorph suppresses superconductivity and results in the a formation of the famous superconducting dome, with a complex phase composition [15]. In contrast, the Tc of the intercalated AxFe2-ySe2 (Aalkali metals) decreases upon application of external pressure, and eventually superconductivity vanishes [16,17]. Upon further pressure increase (P > 11.5 GPa) a new superconducting phase (denoted as SCII) with a Tc reaching 48 K was reported [18]. This result, however, has not yet been experimentally reproduced with independent studies reporting much lower critical temperatures (~5 or ~20 K depending on sample preparation procedures [19,20]).The structural properties of alkali-intercalated FeSe phases are complex, as are the ones of the parent FeSe phase [15]. Firstly, the AxFe2-ySe2 family features intrinsic phase separation and the second minor phase is responsible for the observed supercond...

Intercalated Iron Chalcogenides: Phase Separation Phenomena and Superconducting Properties

“…Despite the observation of phase separation in A 1−x Fe 2 Se 2 , the transport, magnetization, and heat capacity studies confirmed 100% Meissner shielding in these materials, which seems a bit controversial in the context of a phase separation phenomenon evidenced so far by neutron diffraction (Yu Pomjakushin et al, 2012;Zhao et al, 2012), x-ray diffraction and spectroscopy studies (Ricci et al, 2011;Bosak et al, 2012;Svitlyk et al, 2018), muon spin rotation spectroscopy (Charnukha et al, 2012;Shermadini et al, 2012), NMR (Torchetti et al, 2011;Texier et al, 2012), scanning electron microscopy (SEM) (Speller et al, 2012(Speller et al, , 2014, scanning tunneling microscopy (STM) (Ding et al, 2013), and magnetic force microscopy (MFM) imaging (Hazi et al, 2018;Dudin et al, 2019). After nearly a decade of research on the origin of phase separation phenomena, there is a general agreement that the vacancy-free A 1−x Fe 2 Se 2 superconducting phase forms only in the presence of an insulating vacancyordered antiferromagnetic majority phase with the chemical composition close to A 0.8 Fe 1.6 Se 2 (Speller et al, 2012;Carr et al, 2014;Bao, 2015) while the superconducting one is a metallic vacancy-free phase.…”

“…After nearly a decade of research on the origin of phase separation phenomena, there is a general agreement that the vacancy-free A 1−x Fe 2 Se 2 superconducting phase forms only in the presence of an insulating vacancyordered antiferromagnetic majority phase with the chemical composition close to A 0.8 Fe 1.6 Se 2 (Speller et al, 2012;Carr et al, 2014;Bao, 2015) while the superconducting one is a metallic vacancy-free phase. There were also several structural models suggested for both these phases involving the different ordering of iron and alkali metal vacancies, specific ordering of charges and spins, and magnetic domains orientation (Li et al, 2012;Ding et al, 2013;Carr et al, 2014;Svitlyk et al, 2018), but until today any consistent model covering all the experimental observables has not been proposed. It has been experimentally proved that tiny domains of the minority phase are uniformly distributed in the AFM-ordered majority A 0.8 Fe 1.6 Se 2 phase, forming a specific percolation network throughout the whole macroscopic crystallite, enabling high Meissner screening.…”

“…It has been experimentally proved that tiny domains of the minority phase are uniformly distributed in the AFM-ordered majority A 0.8 Fe 1.6 Se 2 phase, forming a specific percolation network throughout the whole macroscopic crystallite, enabling high Meissner screening. The phase separation effect in these materials turns out to be reversible in a relatively narrow temperature range and annealing the crystals at the temperature corresponding to the phase separation, below the temperature of the iron vacancies ordering, causes a microscopic reorganization of plates of the minority phase resulting in superconducting properties of these materials (Bosak et al, 2012;Weyeneth et al, 2012;Yu Pomjakushin et al, 2012;Speller et al, 2014;Svitlyk et al, 2018). Moreover, from the available experimental data, it appears that the temperature range of the phase transitions depends primarily on the concentration of the alkali metal and not on the ionic radius of the intercalating ion.…”