Microscopic origin of the magnetoelectric coupling in the all-organic radical-ferroelectric composite of potassium-doped terphenyl ($K_{1.5}C_{18}H_{14}$) and imidazolium perchlorate

The 2019 report of 'magnetoelectric radical hydrocarbons' presented a pressed all-organic composite in which a potassium-intercalated p-terphenyl radical phase ($K_{1.5}C_{18}H_{14}$) and the molecular ferroelectric imidazolium perchlorate ($\mathrm{ImClO_4}$) show co-located anomalies in resistivity, dielectric constant, and magnetization near 250 K, an electric-field-induced magnetization change at room temperature (magnetoelectric coefficient of roughly 2.9 V Oe$^{-1}$ cm$^{-1}$), and a photoenhanced positive magnetoconductance attributed to magnetic-field-driven singlet-triplet intersystem crossing. None of the microscopic mechanisms behind these observations has been established: the magnetic ground state of the radical phase rests on an unvalidated density-functional prediction whose antiferromagnetic-ferromagnetic energy splitting (about 1.8 meV) lies within functional error, the coupled 250 K anomaly has never been tested against a structural transition, and no control experiment (non-ferroelectric analog, poling dependence, time-resolved spin spectroscopy) discriminates the proposed coupling and magnetoconductance mechanisms from strain-mediated or trivial alternatives. No later work has resolved any of these questions. The problem is to determine, by microscopic experiment and discriminating controls, the magnetic ground state and ordering of the $K_{1.5}C_{18}H_{14}$ phase and the origin of the correlated 250 K anomaly, the causal pathway by which ferroelectric polarization and radical spin order couple in this composite, the mechanism of the photoenhanced positive magnetoconductance, and the microscopic origin of the reported photocurrent and photovoltage, as set out in the linked questions below.

Background

A molecular radical is a molecule with an unpaired electron, and radical solids can carry localized spins that interact through magnetic exchange. The magnetoelectric effect is the cross-control of magnetization by electric fields and of polarization by magnetic fields; it is most often sought in multiferroics, materials that combine magnetic and electric order, and in composites that couple a ferroelectric (a solid with switchable electric polarization) to a magnetic phase. Almost all practical magnetoelectrics are inorganic ceramics, so an all-organic, metal-free magnetoelectric would be lightweight, flexible, and processable at low temperature.

Guan et al. (Advanced Materials 31, 1806263, 2019) reported such a candidate: a pressed composite (called K-Im, mixed roughly 1:2) of two organic molecular solids. The first component is potassium-intercalated para-terphenyl, $K_{1.5}C_{18}H_{14}$: p-terphenyl (C18H14, three benzene rings linked in a chain) doped with potassium, which transfers charge to the molecules and creates stable terphenyl radical anions, evidenced by electron paramagnetic resonance (EPR, a spectroscopy that detects unpaired spins; an isotropic signal at g = 2.0029 with partially resolved hyperfine structure at 77 K) and by optical absorption bands between 572 and 940 nm. The second component is imidazolium perchlorate ($\mathrm{ImClO_4}$), a known order-disorder molecular ferroelectric that crystallizes in the non-centrosymmetric space group R3m with a ferroelectric transition near 373 K. Within the composite, piezoresponse force microscopy (PFM, an atomic-force technique imaging switchable ferroelectric domains) showed reversible domain switching of the $\mathrm{ImClO_4}$ phase, and scanning electron microscopy with elemental mapping showed micron-scale phase-separated conductive (radical) and insulating (ferroelectric) regions with well-defined interfaces.

The reported observations were: (i) a correlated anomaly of four-probe resistivity, dielectric constant, and magnetization centered near 250 K, interpreted as a coupling between magnetic and charge orders; (ii) a rise of magnetization at the 373 K ferroelectric transition of $\mathrm{ImClO_4}$ and a reversible electric-field-induced magnetization change at room temperature, quantified by a magnetoelectric coefficient of about 2.9 V per Oersted per cm; (iii) a substantial steady photoconductivity (of order 0.2 S/cm) and photovoltage under visible light, attributed to interfacial ferroelectric bound charges assisting the separation of photogenerated excitons (bound electron-hole pairs); and (iv) a positive magnetoconductance that grows under photoillumination, interpreted as a magnetic-field-driven intersystem crossing (spin-state mixing between singlet and triplet radical-pair states, a mechanism familiar from organic spintronics) that increases the conducting spin population.

Each of these interpretations rests on circumstantial evidence. The antiferromagnetic ground state assigned to $K_{1.5}C_{18}H_{14}$ comes from density-functional calculations (vdW-DF2 functional) that place the antiferromagnetic state only 1.8 meV (equivalent to about 21 K) below the ferromagnetic one, within the systematic uncertainty of such functionals for molecular radicals. No variable-temperature X-ray diffraction or Raman measurement was performed, so a structural, rather than magnetic, origin of the 250 K anomaly was never excluded. The magnetization changes (of order $4\times10^{-5}$ emu/g) sit near typical impurity and instrument backgrounds, and no control composite with a non-ferroelectric insulator was measured. The intersystem-crossing picture was asserted without time-resolved EPR or magneto-optical data, and ordinary positive magnetoresistance in a disordered conductor was not ruled out. Adjacent literature sharpens rather than settles the questions: claims of 7-123 K superconductivity in potassium-doped p-terphenyl were never reproduced and concern different phenomena, the only structurally characterized magnetic order in this materials family is a crown-ether-stabilized stoichiometric K-doped p-terphenyl antiferromagnetic insulator that orders at 6 K, and the recently developed high-temperature supramolecular radical ferroics are ferroelectric but paramagnetic, with non-interacting spins. Whether a genuine, magnetically ordered, all-organic radical magnetoelectric exists therefore remains an open experimental question concentrated on this composite.

Problem Statement

Establish the microscopic origin of the coupled magnetic, dielectric, and transport behavior of the all-organic radical-ferroelectric composite of potassium-doped p-terphenyl ($K_{1.5}C_{18}H_{14}$) and imidazolium perchlorate ($\mathrm{ImClO_4}$) (the K-Im composite of Guan et al., Adv. Mater. 31, 1806263, 2019). The single scientific objective is to replace the paper's inferred mechanisms with microscopic evidence and discriminating controls; the following linked questions jointly define completion, because each addresses one facet of the coupled spin, charge, and polarization behavior of the same composite:

1. Magnetic ground state and the 250 K anomaly. Determine the magnetic ground state of the $K_{1.5}C_{18}H_{14}$ radical phase in the composite (nonmagnetic, ferromagnetic, or antiferromagnetic; ordered moment and ordering temperature), and establish whether the correlated anomaly in resistivity, dielectric constant, and magnetization near 250 K is driven by magnetic ordering rather than by an unreported structural phase transition of either component. The existing density-functional prediction of an antiferromagnetic ground state rests on a reported antiferromagnetic-ferromagnetic energy splitting of about 1.8 meV that lies within functional error and needs experimental validation or refutation.

2. Causal pathway of the magnetoelectric coupling. Identify the mechanism by which applied electric fields (and pressure) change the composite's magnetization, and by which the ferroelectric transition of $\mathrm{ImClO_4}$ at about 373 K affects it: direct coupling between radical-radical exchange interactions and charges accumulated at the radical-ferroelectric interfaces, indirect coupling mediated by strain or the piezoelectric response of $\mathrm{ImClO_4}$, or another route. This requires controls such as a non-ferroelectric dielectric substituted for $\mathrm{ImClO_4}$, poled (polarization pre-aligned by applying a strong electric field) versus unpoled samples, and exclusion of Joule-heating and magnetic-impurity artifacts in the magnetization measurements.

3. Mechanism of the positive magnetoconductance and its photoenhancement. Determine whether the observed field-induced conductivity increase and its enhancement under visible-light illumination arise from a magnetic-field-driven singlet-triplet intersystem crossing acting on an increased radical population (as proposed), or from alternatives such as a partial antiferromagnetic-to-ferromagnetic rearrangement, a field-lowered transport barrier, or ordinary magnetoresistance of a disordered conductor, using direct probes such as time-resolved or field-dependent EPR and quantified magnetoconductance field dependences.

4. Origin of the photocurrent. Quantitatively separate the contribution of interfacial ferroelectric polarization fields of $\mathrm{ImClO_4}$ to the steady photocurrent and photovoltage from thermal, pyroelectric (current caused by temperature-induced change of the polarization of a polar crystal), and bulk-photoconductive contributions of the separate components, for example by poling-dependence and non-ferroelectric-control measurements.

The problem is considered solved when a coherent microscopic account, supported by analysis of the four-probe resistivity, dielectric, and magnetization data reported by Guan et al. (2019) and by a discriminating experimental program of measurements and controls with predicted outcomes, answers all four questions consistently for this composite.

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