Tracing the horizon of tetragonal-to-monoclinic distortion in pressurized trilayer nickelate La4Ni3O10

Abstract

The crux of understanding the superconducting mechanism in pressurized Ruddlesden-Popper nickelates hinges on elucidating their structural phases. Under ambient conditions, the trilayer nickelate La4Ni3O10 stabilizes in a twinned monoclinic structure with space group P21/c. Upon heating, it undergoes a structural transition to the tetragonal I4/mmm phase at Ts ~ 1030 K, while a second transition associated with the onset of density-weave (DW) ordering emerges upon cooling below TDW ~ 135 K. Here from pressure-temperature x-ray diffraction on high quality flux-grown single crystals we unequivocally demonstrate a direct tetragonal-to-monoclinic transition with no trace of intermediate orthorhombic Bmab phase. Ab initio density-functional theory calculations as a function of pressure fully corroborate the experimental observations. The transition unfolds as a 2-fold superstructure due to the emergence of commensurate superlattice reflections and can be progressively suppressed from 1030 K down to 20 K under 14 GPa. No discernible structural distortions associated with DW ordering are detected down to 20 K at ambient pressure. This is in contrast to Raman measurements that reveal the appearance of additional phonon modes below 130 K, implying a further reduction in symmetry from monoclinic P21/c and thus indicating the presence of a third structural phase associated with the DW ordering in La4Ni3O10.

Figures (6)

• Figure 1: Pressure-temperature phase diagram illustrating the tetragonal-to-monoclinic structural transition. Dashed line serving as a visual guide to indicate the linear suppression and signifies the phase boundary between 4/$mmm$ and 2/$m$ symmetries. The present result of XRD above 1000 K closely matches the work done in the past nagell2017aSong2020a. Our complementary $ab$-initio calculations predict a critical pressure point $P_c$$\approx$ 16 GPa at 0 K, in good agreement with experimental observations, thereby reinforcing its validity. Bragg reflections at room temperature and at 80 K are also shown to illustrate their evolution from $4/mmm$ to $2/m$. Upon entering the monoclinic phase they tend to broaden and become anisotropic.
• Figure 2: (a). Reconstructed reciprocal layer showing specific reflections indexed in non-standard settings in the $hk0$ plane at 300, 80 and 20 K. At on the onset of the structural transition from $4/mmm$ to $2/m$, the reflection becomes broader and stretched and later splitting into two at lower temperatures 80 and 20 K, indicative of pseudo-merohedral twinning resulting from the monoclinic distortion of the lattice. (b). Excerpts of the $hhl$ plane where yellow circles indicate superlattice reflections at q = $(\frac{1}{2}, \frac{1}{2}, 0)$ when indexed on the HP (High Pressure) standard-lattice that corresponds to an $I$-centered pseudo-tetragonal lattice where $a$ = $b$$\approx$ 3.7 Å, $c$$\approx$ 27 Å. (c) Panels of the reciprocal layers of the $hk$-1 plane of the LP (Low Pressure) non-standard-lattice where $a$$\approx$ 5.41 Å, $b$$\approx$ 5.43 Å, $c$$\approx$ 27 Å and $\beta \neq 90^\circ$, showing presence of twinning due to weak diffraction spots indicated by yellow circles below 11.3 GPa. The lattice is $F4/mmm$ at 11.3 GPa and below that it reduces to $B2_1/a$.
• Figure 3: Crystal structures of La$_4$Ni$_3$O$_{10}$. (a) The high pressure phase (15 GPa) at room temperature where the crystal structure is tetragonal. The $F4/mmm$ structure whose unit cell indicated by thick lines is superimposed onto the standard $I4/mmm$ structure to have a visual representation of the transformation between the two, where bond angle of Ni1-O2-Ni2 is 180$^\circ$. (b) Monoclinic phase at room temperature 6 GPa. Here the $B2_1/a$ structure (thick lines) has been drawn onto the standard $P2_1/c$ structure (thin lines) to show the transformation between the two visually, where bond angle of Ni1-O2-Ni2 is 171.1$^\circ$. The mathematics of the transformations are described in the SI la4ni3o10suppmat2025a. (c) Evolution of the octahedral tilt as a function of pressure at 300 and 80 K. Dashed lines are just a guide to the eye.
• Figure 4: (a)--(d) Lattice parameters, distortion and the volume within the pressure range of 0--16 GPa for temperatures at 300, 80 and 20 K. In the tetragonal phase (4/$mmm$), the lattice parameters correspond to the non-standard $F$-centered setting of $I$4/$mmm$ and for the monoclinic phase (2/$m$) we use the $B$2$_1$/$a$ non-standard space group in order to preserve the same setting of the lattice which allows for a comparison. Error bars are smaller than the symbols which can be understood from the numerical values listed in Table S5 in the SI la4ni3o10suppmat2025a. Fits of a Birch-Murnaghan equation of state (EOS) to the volume-pressure data has resulted in V$_0$ = 825(1) Å${^3}$ and K$_0$ = 155(4) GPa at 300 K, V$_0$ = 818(1) Å${^3}$ and K$_0$ = 182(4) GPa at 80 K, and V$_0$ = 815(1) Å${^3}$ and K$_0$ = 185(2) GPa at 20 K. See the SI for details la4ni3o10suppmat2025a.
• Figure 5: Density-functional theory GGA PBE crystal relaxation as a function of applied external pressure: red dots and lines: basal distortion $(b-a)/(a+b)$ to be scaled by $10^3$ (red leftmost $y$ axis); blue dots and lines: NiO octahedra tilt angle $\theta = 180 -$ Ni1-O2-Ni2 in degrees (blue rightmost $y$ axis); monoclinic angle $\beta - 90$ in degrees (black middle $y$ axis placed at $P_c$). Both horizontal and vertical error bars are smaller than the dot size when not visible. Dashed lines refer to fits, whereas dotted lines are just only a guide to the eyes.