Bond-Only Hydride Frameworks for Ambient-Pressure Superconductivity: A Calibrated Screening Model, Three Candidate Families, and Two Falsifiable Predictions

The search for superconductivity above 150 K at ambient pressure has so far proceeded without a compact, experimentally calibrated screening instrument for conventional superconductors. This paper constructs such an instrument, applies it, and extracts testable consequences. The model is the complete Allen–Dynes form of the McMillan equation, with the Coulomb pseudopotential assigned by a screening-based tier scheme and the spectral second moment taken from the phonon distribution of each material class. Calibrated against twenty-eight known superconductors spanning 0.9 K to 252 K — elements, A15 compounds, MgB₂, transition-metal compounds, fullerides, ambient-pressure hydrides, high-pressure hydrides, and doped semiconductors — the model reproduces the experimental transition temperatures with a root-mean-square logarithmic error of 0.105, with 80% of materials predicted within −14% to +8%. Three design rules follow. First, carriers must occupy bond orbitals rather than lone pairs: only XHₙ anions in which the central atom's valence electrons are fully invested in X–H bonds (BH₄⁻, AlH₄⁻, GaH₄⁻, InH₄⁻) can translate high-frequency stretching modes into pairing, while SH⁻, NH₂⁻, OH⁻, and CH₃⁻ cannot. Second, the logarithmic phonon frequency is the binding constraint at 300 K, not the coupling strength: λ ≥ 2.2 has been reported at ambient pressure in three independent systems, whereas ω_log ≥ 2200 K has none. Third, narrowing bands to raise λ raises μ* in tandem, leaving a single narrow corridor for 300 K. Applying these rules produces three concrete propositions: (i) alkali-hole-doped alkaline-earth borohydrides, M₁₋δKδ(BH₄)₂ with M = Sr, Ba — predicted transition temperatures of 155–241 K, with a 300 K corridor at 325–363 K and an estimated probability of 3–8%; (ii) the alanate analogues M(AlH₄)₂ with alkali substitution; and (iii) pressure-quench processing of the 298 K superhydride LaSc₂H₂₄, whose quenched transition temperature is estimated at ≈274 K.